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      09-21-2017, 11:14 PM   #1
dsc888
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2017 340ix Knocking Noise When Gas Tank is Below 1/4 Full

Hi Guys,

I picked up my new 2017 340ix 2 weeks ago and it's making a knocking noise from the undercarriage when the fuel tank is below 1/4 full. (I know that most people don't recommend running with so little gas in the tank but there are times when I am not near a gas station.)

I noticed there are NO drivability issues. No Check Engine lights. Engine RPMs are stable. I hear it while stopped in Drive or when the car is in Park. I can hear it slightly as I am coasting very slowly also. So it's NOT the drivetrain. It's clearly from the exhaust or the cats. Here's my a video that I shot of the noise.



As I googled this issue, I also found another 340i that had the same issue. Another viewer of this video also chimed in and said his car had the same issue and the dealer said it's related to the cats "cooling off". The poster said that his noise also occurred with the tank below 1/4 tank as well.



Does anyone here have this issue? If so, did you ever get it resolved?

Last edited by dsc888; 09-21-2017 at 11:19 PM.
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      09-21-2017, 11:22 PM   #2
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I have the same issue. I actually noticed it since my first tank of gas from delivery.

For me, it only happens when I'm <1/8 of a tank.

Didn't get it checked out. I just fill up my tank and the noise goes away. I've learned the 340 (I suppose the F30 in general) has a bunch of annoying creaks, rattles, and knocks. I just live with it at this point.
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      09-21-2017, 11:33 PM   #3
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I would raise the car enough to get under it while running and use a mechanic's stethescope to find the source of the noise
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      09-21-2017, 11:55 PM   #4
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Quote:
Originally Posted by thisberob View Post
I have the same issue. I actually noticed it since my first tank of gas from delivery.

For me, it only happens when I'm <1/8 of a tank.

Didn't get it checked out. I just fill up my tank and the noise goes away. I've learned the 340 (I suppose the F30 in general) has a bunch of annoying creaks, rattles, and knocks. I just live with it at this point.
Wow! Now this is making me wonder if BMW didn't do enough testing when they engineered the 340i. And yes, I was between 1/8 to 1/4 tank when it happened the first time which was when the dealer full tank of gas was almost gone. Then it happened again today a week and a half later. It was gone right after I filled up. So the key is to keep it above 1/4 tank full BUT this noise is bothering me. My 2015 335ix that I traded in didn't have this noise.
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      09-21-2017, 11:57 PM   #5
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Quote:
Originally Posted by rak299 View Post
I would raise the car enough to get under it while running and use a mechanic's stethescope to find the source of the noise
Great idea! It seems to get louder as I approach the cats near the front/center of the car but it's not in the engine bay or the very back from the exhaust/muffler.
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      09-22-2017, 08:25 AM   #6
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If you think about it, when the tank is nearly empty, it gets lighter and isn't being weighed down by the fuel. Wonder if it has something to do with clearance between the fuel tank and something above or near it on the chassis or suspension or even the exhaust system? I'd be taking it back to the dealership - it has ZERO to do with the catalytic converters "cooling off" - that is generally heard as a series of popping/creaking noises for a minute or two after the engine is shut off and the cats and pipes start cooling and contracting (perfectly normal for exhaust components). It's definitely not due to "lack of testing by the BMW Engineers".
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      09-22-2017, 09:46 AM   #7
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Quote:
Originally Posted by thisberob View Post
I have the same issue. I actually noticed it since my first tank of gas from delivery.

For me, it only happens when I'm <1/8 of a tank.

Didn't get it checked out. I just fill up my tank and the noise goes away. I've learned the 340 (I suppose the F30 in general) has a bunch of annoying creaks, rattles, and knocks. I just live with it at this point.
I feel you. When I leave something in the trunk and it rolls around as I drive, the sound is so loud and obvious, sounds like something is rolling around above my head. Horrible sound insulation.
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      09-22-2017, 10:57 AM   #8
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I have the EXACT same sound in my 330i when the tank is at 1/4 or below. Since this happens with a different engine and exhaust layout, could it actually be something in the fuel tank that's rattling?
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      09-22-2017, 11:17 AM   #9
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this is funny and strange because ive always had this issue with my old 1987 VOLVO 740 GLE. that i havent had in over a decade now. But every time the tank got low, it would make strange noises.
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      09-22-2017, 11:32 AM   #10
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I'd say that when the tank is approaching empty it becomes a resonator, amplifying chassis transmitted vibrations to a higher than normal level.
Aside from strange noises you shouldn't let the tank go below 1/4, as it increases the potential for water condensation in the tank.
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      09-22-2017, 01:04 PM   #11
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Quote:
Originally Posted by roundel335 View Post
If you think about it, when the tank is nearly empty, it gets lighter and isn't being weighed down by the fuel. Wonder if it has something to do with clearance between the fuel tank and something above or near it on the chassis or suspension or even the exhaust system? I'd be taking it back to the dealership - it has ZERO to do with the catalytic converters "cooling off" - that is generally heard as a series of popping/creaking noises for a minute or two after the engine is shut off and the cats and pipes start cooling and contracting (perfectly normal for exhaust components). It's definitely not due to "lack of testing by the BMW Engineers".
I have to agree. I had cars with the pops and crackles before so I highly doubted the idea that it was the cats, as reported by another owner. I will ask the dealer when I get a chance and show them the videos.
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      09-22-2017, 01:07 PM   #12
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Quote:
Originally Posted by KGB_123_99 View Post
I have the EXACT same sound in my 330i when the tank is at 1/4 or below. Since this happens with a different engine and exhaust layout, could it actually be something in the fuel tank that's rattling?
Thank you for chiming in! That is very interesting. I wonder now if it is isolated to the LCI F30s only. I've never come across any threads regarding this noise. Seems like all folks affected have an LCI model. Hmmm. I wonder if there were underbody changes or even a fuel tank design change that is now causing the noise when almost empty.
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      09-22-2017, 01:10 PM   #13
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Quote:
Originally Posted by Billfitz View Post
I'd say that when the tank is approaching empty it becomes a resonator, amplifying chassis transmitted vibrations to a higher than normal level.
Aside from strange noises you shouldn't let the tank go below 1/4, as it increases the potential for water condensation in the tank.
That actually makes a lot of sense. The weight of the fuel essentially dampens the resonances the run through the car from the exhaust due to the proximity

I travel for work and sometimes try to get my work done before stopping for gas so I will let it run low just for the day and fill up when I get off work. But I agree that it's not a good idea to let the fuel run too low. And now, it appears I have a audible "reminder" under the car to let me know each time.
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      09-22-2017, 01:11 PM   #14
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No noise in my 2014 335. Thankfully! It does seem like it's LCI only? That's a weird issue. Haven't heard much about issues like that before.
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      09-23-2017, 04:28 AM   #15
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It looks like this issue is getting noticed by owners. Here’s another owner of a 2017 330ix who found my video and posted her own.



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      09-23-2017, 08:57 AM   #16
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I raised my car on jack stands yesterday and got under it to figure out where it is coming from. I thought it might be the exhaust hitting the gas tank but I can move the latter freely and it doesn?t touch anything.

I took a screwdriver, put it on the gas tank and on my ear. It really sounds like it is coming from inside the tank! Could it be the fuel pump???

I also notice that there is no noise even when the tank is empty when the car is cold. It needs to run for a while before the sound appears.
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      09-23-2017, 09:13 AM   #17
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Quote:
Originally Posted by KGB_123_99 View Post
I raised my car on jack stands yesterday and got under it to figure out where it is coming from. I thought it might be the exhaust hitting the gas tank but I can move the latter freely and it doesn?t touch anything.

I took a screwdriver, put it on the gas tank and on my ear. It really sounds like it is coming from inside the tank! Could it be the fuel pump???

I also notice that there is no noise even when the tank is empty when the car is cold. It needs to run for a while before the sound appears.
So it would appear that could be temperature-related, in that some component warms up and expands and causes the "rapping" sound. It is possible that it's the fuel tank, but I wouldn't think temperature would affect the submerged pump. I still think it may be something above the fuel tank where you can't really reach. I'd have the service dept. at your dealer remove the tank or at least release it from its mounts so they can see if there's something protruding from the bottom of the chassis that might be contacting it.
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      09-23-2017, 11:05 AM   #18
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Quote:
Originally Posted by KGB_123_99 View Post
I raised my car on jack stands yesterday and got under it to figure out where it is coming from. I thought it might be the exhaust hitting the gas tank but I can move the latter freely and it doesn?t touch anything.

I took a screwdriver, put it on the gas tank and on my ear. It really sounds like it is coming from inside the tank! Could it be the fuel pump???

I also notice that there is no noise even when the tank is empty when the car is cold. It needs to run for a while before the sound appears.
Interesting. Were you able to feel the vibrations on the tank when you were underneath with your hands?
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      09-23-2017, 05:56 PM   #19
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Definitely not cats cooling or any garbage like that. I would venture a guess that it's an echo chamber effect in the near empty tank, or even more likely the fuel pump struggling to pump the lower level of fuel in the tank. Perhaps a mix of both, where the fuel pump is more easily heard in a near empty tank versus submerged in fuel. The consistency in the thumps make me believe it's a mechanical noise, hence the fuel pump. Since it seems common I wouldn't worry, but the E90/92 had poor fuel pumps so it's not necessarily shocking that BMW beefed up the units, maybe making it noisier?
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      09-23-2017, 06:32 PM   #20
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Quote:
Originally Posted by dsc888 View Post
Quote:
Originally Posted by KGB_123_99 View Post
I raised my car on jack stands yesterday and got under it to figure out where it is coming from. I thought it might be the exhaust hitting the gas tank but I can move the latter freely and it doesn?t touch anything.

I took a screwdriver, put it on the gas tank and on my ear. It really sounds like it is coming from inside the tank! Could it be the fuel pump???

I also notice that there is no noise even when the tank is empty when the car is cold. It needs to run for a while before the sound appears.
Interesting. Were you able to feel the vibrations on the tank when you were underneath with your hands?
Couldn?t feel the vibrations but hitting the tank with my knuckles produced a similar sound.

I am due for an oil change in a week or so and will tell my SA about it. Will report back.
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      09-23-2017, 06:41 PM   #21
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Quote:
Originally Posted by KGB_123_99 View Post
Couldn?t feel the vibrations but hitting the tank with my knuckles produced a similar sound.

I am due for an oil change in a week or so and will tell my SA about it. Will report back.
Thanks for the reply. And definitely share your findings with the forum. I hope you can go there with your tank on the low side so they can hear it and give you a diagnosis. I'm sure BMW engineering will get plenty of feedback as customers begin to complain about the same issue. Maybe we will even see a Service Bulletin in the months to come.
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      09-23-2017, 06:46 PM   #22
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Quote:
Originally Posted by TheLeusOne View Post
Definitely not cats cooling or any garbage like that. I would venture a guess that it's an echo chamber effect in the near empty tank, or even more likely the fuel pump struggling to pump the lower level of fuel in the tank. Perhaps a mix of both, where the fuel pump is more easily heard in a near empty tank versus submerged in fuel. The consistency in the thumps make me believe it's a mechanical noise, hence the fuel pump. Since it seems common I wouldn't worry, but the E90/92 had poor fuel pumps so it's not necessarily shocking that BMW beefed up the units, maybe making it noisier?
That's definitely a possibility. I'm just surprised that pre-LCI cars didn't have this issue if they decided to spec a heavier duty unit. Going by your theory, BMW might have gone with a different pump for the 2017s possibly as only 2017 cars seem to be reporting this issue with the few instances that's been found here and on YouTube. And indeed, a common problem puts my mind at ease as it might simply be characteristic of these recent cars. I'd like to hear what the techs say about the issue. I will definitely make an appointment to have it looked at if it caused drivability issues. But for now, I might hold off till my first oil change.
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