[Skyhigh]

I am very puzzled, had this happen first an year ago:
- winter conditions, lots of water on the road.
- normal cruising
- during light (~10%) throttle, around 2000-2300 RPM.
- independent from the speed

Loud rattle and noticeable vibrations in the chassis.

MPPSK + Akrapovic Downpipe

Today it was snowing, slush on the road. Same thing. Previously I though it is coming from the front-right wheel somehow, during light acceleration (never without throttle), now I am pretty sure it is related to the exhaust, maybe the downpipe connections... still front-right direction or under the car.

It is not easy to reproduce, it doesn't happen every time, but often enough in those conditions. I wasn't able to record it unfortunately.

2000-2500 RPM is where the exhaust is the loudest and has some drone, but that rattle is something that comes in addition, most likely related...
It sounds as if a thick sheet metal or plastic is being bombarded with wet snow. One feels the vibration from the rattle. It fades away as RPMs increase above about 2500.

If it is the exhaust however, I cannot understand the link to water on the road / slush!?!?

Any theories or experience??

[Wires]

Are you sure your DP/midpipe aren't rubbing on the frame? in slush the engine will work harder, and will want to rotate more on the mounts.

When I did my VRSF, it was twisted rubbing the engine under hard acceleration.

[Skyhigh]

No, I'm not. From what I can see from above it is not, but I haven't inspected the car on a lift yet. The DP is attached with it's attachment bracket to the frame, I can see that from above,

I am not a big believer in the theory about engine needing to work harder in water/slush to the extent to twist out of the ordinary... And only in that RPM range, not under harder acceleration, not even under WOT It does not seem to happen on snow either. It really requires gentle acceleration or throttle just enough to maintain speed.
I wish I had another or better theory though

The downpipe was installed a few months before I heard the rattle for the first time. I have not heard it before, with the stock DP. The car has been lowered as well, not too long before it fist happened, but as of today, I am more and more certain it has to do with the exhaust and not the wheel as initially assumed. The rattle comes from down-right, seen from the driver's position. I remember last year I thought it is coming from just under the passenger door... it is very crisp and distinct.

[Wires]

You can see if it's the midpipe by shining a flash light forward from the back of the car and follow the exhaust up.

Maybe it's close enough that slush and stuff gets tossed up there and creates a sound coupler before it melts off? Pure conjecture here....

[Skyhigh]

Where would the tricky part be? What should I be looking for?

The exhaust should IMO be warm/hot enough that no slush or snow would ever stick around it.

[njdangelo]

Any possibility it had something to do with the handbrake cable? I know that makes a hell of a noise. Maybe It's a perfect storm sort of situation with the water and the native exhaust vibration? That whole water/slush aspect makes this quite the conundrum.

[Skyhigh]

No (unlikely). I know about this weak spot, and from what I have heard on videos here, the sound is nothing like it. The handbrake cable is high frequency rattle, the one I am experiencing is a deeper, low frequency rattle which can be felt very clearly in the chassis. It creates very perceivable vibrations, so it's an item of mass.

Best way to imagine it is a thicker sheet metal or plastic which is put in resonance.

Today I had a short drive in some slush and wet roads and since I could not reproduce it at all, not even close, reached the conclusion water/slush needs to first accumulate somewhere before the phenomena happens. Just driving on water or slush does nothing. It needs to be a lot of it and accumulate somewhere... something maybe gets heavier this way and either rubs or acquires a different natural resonance frequency... This could explain why it disappears at some stage during the ride and is hard to reproduce (if water is being "shaken off" or goes out during acceleration and then re-accumulates, during gentle cruising...

It could be my imagination, but yesterday I also got the impression the exhaust generally gets louder under watery conditions and the normal drone around 2000-2300 rpm gets louder. I could be wrong though.

[speedbirdblack]

I also experience the exacty same thing after engine rebuild (snapped timing chain). Only happens when it rains at low rpm.(heavy rattling and vibration) As soon as the rain goes away, the rattle disappear. Also noticed slight rattle at cruising at 100kph arround 1200rpm. (Engine is warm) nothing at high rpm.

[Phoenix9110]

For anyone who finds this post. When you hear the droning, it's due to the slush actually being packed in the wheel well causing the tire to drag. I had it happen for the first time ever the other day and my wheel well was packed with snow and slush. I snowboard every year at the local mountain and drive through most conditions, even traveling to other mountains.
Weird that it hasn't happened before. Oddd thing to see. Odd thing to search if other people had the same issue. Just pointing out the cause of the issue.

Edit: My F30 has zero issues. Stage 1 with BM3. Monitored regularly. It definitely stood out as a 'wtf is that' sound.

[Skyhigh]

That’s definitely not it in my case (ref original post). I am now 100% sure mine is related to the exhaust somehow (directly or indirectly), as it only happens during moderate acceleration (never ever during idle) and it only started happening after the DP upgrade….

[Burnout]

I get a higher frequency rattle from the back end in slushy/snowy conditions.

I suspect it’s ice forming on the upper surfaces of the plastic air baffles, but haven’t yet done a more extensive investigation.

OPI realize you’ve got a lower frequency vine but perhaps the mechanism is the same but the affected location is different

I’ve got a 335xi.

[camluczi]

I have a very similar issue!!

Noise when there is lots of water or slush. Not all the time, but usually at higher speed (>80kmh) and only lasts for a few seconds before disappearing. I had this happen on different wheels and tires in the summer when there was lots of water in the rut of the road, and in the winter when there is slushy snow build up and changing lanes.

Stock 340 xdrive with M Performance exhaust.

Everything seems tight suspension wise - it seems / sounds like its coming from the passenger side wheel to me, but a passenger that heard it said it sounded like it was coming more central footwell area.

The only thing I could find, was just behind the front lower control arm (camber arm), there is a spot where the front undertray and side undertrays overlap. there is a bit of overhang here, so it could be that snow/water get in there and it opens up even more and with the airflow in the wheel well, starts to vibrate. It almost sounds like going over rumble strips when it makes the noise.

I put unbolted the back part of the undertray and wedged a foam piece in between and then tightened it back down. Have not been able to replicate the noise again since, but that was just a few days ago and the roads are clear right now.

The only thing that I cannot figure out, is that if it makes the noise and I let off the throttle, it totally disappears again. If I go back on the gas, even just slightly, it starts again... Then as always, if I leave on the throttle, it disappears on its own after a few seconds. Very weird.

I was thinking it might be front diff, front driveshaft, or center diff - but then it never happens in the dry. I did try on snowy roads and giving it loads of throttle at speed, the traction light going off like crazy and it wouldnt make the noise...

[Skyhigh]

camluczi

Might sound evil, but I am glad there is finally someone with the exact same issue as me!! Your description is a 100% accurate description of what I have been experiencing!!

- I also hear it from the passenger front wheel or under the passenger feet.
- It ONLY happens in heavy rain / slush
- Only at light throttle and RPM. Foot away from the throttle - gone. Foot heavier on the throttle - it disappears in a second or two and does not come back until I drive gently for a while (and snow/slush/water accumulates somewhere). This is why it is not easily reproducible even in the right weather… something accumulates somewhere during gentle driving and is triggered from the specific vibrations of the exhaust or engine at certain low RPMs, until the extra weight gets shaken off. Then it is gone again until it reloads. Never heard during sporty driving or high rpms.

Ironically, last summer I discovered that one of the plastic covers just aft of the front-right wheel, where there is an overlap (probably the same spot you are referring to) was missing a fastener and could be oscillated freely when touched. I was 200% sure I have found and fixed the cause as the noise it made once triggered was loud and familiar and the location made perfect sense! Until 2 weeks ago, when in slush/rain I heard it again…. 😖😖

I am waiting for warmer days to inspect the area once again and look for other potential triggers. Please do report about your fix!

In my case I first heard the noise after DP upgrade so I assume it has something to do with the installation (maybe a heat-shield?). I have MPPSK too, but have not heard the noise prior to the DP upgrade. (The missing fastener was definitely also a result of the DP installation, I assume…)

[Skyhigh]

camluczi Any luck finding the cause?

[brosot07]

I just had this experience an hour ago. Light throttle it vibrates underneath the chassis. I get a feeling this maybe my bottom metal sheet cover thing accumulating water and somehow is hitting one of these components (mid pipe suspectedly).

At first i thought water sipped thru my engine, but if that was the case the warning wouldve turned on, and the sound didnt come from afront but rather below. Ill have mine lifted up and checked.

ps: was browsing the forum when the sound happened lmao

[Skyhigh]

Welcome to the club 🙂
Was it heavy rain/water or slush…? As soon as you “shake it off” it goes away for a while?

[brosot07]

Originally Posted by Skyhigh

Welcome to the club 🙂
Was it heavy rain/water or slush…? As soon as you “shake it off” it goes away for a while?

I didnt dare to hit the throttle when it was vibrating so I was just cruising with the speed as is. After awhile it went away drove normally.

[Skyhigh]

Did a thorough inspection of the undercarriage. Nothing suspicious, nothing out of the ordinary, all screws in place, all panels properly seated...

My primary suspect now is this asymmetric plastic cover under the gearbox! Although it wasn't that wet or rainy today, it was dripping water from the backside. It apparently gets plenty of on its upper side. It is large and flat enough to accumulate slush/snow/water during cruising in such conditions and get heavy... until it gets triggered in resonance and shakes off the load.... and then accumulates new....
But not sure what to do about it, as it as as per design... wonder why others are not complaining from this noise, if my theory is correct...

One can see that the opening for the oil drainage is very convenient for water and slush to get in over that cover.

[camluczi]

Originally Posted by Skyhigh

Did a thorough inspection of the undercarriage. Nothing suspicious, nothing out of the ordinary, all screws in place, all panels properly seated...

My primary suspect now is this asymmetric plastic cover under the gearbox! Although it wasn't that wet or rainy today, it was dripping water from the backside. It apparently gets plenty of on its upper side. It is large and flat enough to accumulate slush/snow/water during cruising in such conditions and get heavy... until it gets triggered in resonance and shakes off the load.... and then accumulates new....
But not sure what to do about it, as it as as per design... wonder why others are not complaining from this noise, if my theory is correct...

One can see that the opening for the oil drainage is very convenient for water and slush to get in over that cover.

I havnt had the noise since my "fix" of putting some stiff foam inserts between the overlap area of the undertrays by the back of the wheel arches... until yesterday.

Drove over a small slush area when changing lanes on the highway and had the noise for about 2 seconds

Your suspicion of the trans cover may be accurate - but then you would think more people would have this issue. If I recall, it did start happening after I removed that panel to perform trans fluid flush. I did also notice that the felt sound lining that goes around the transmission tunnel isnt secured on the driver side and was peeling off a bit from the body and it was actually almost touching the driveshaft for the transfer case to the front diff. Could it be that material is getting in there and pushing it further and touching the driveshaft? I can see marks on that driveshaft so I know it has/had been rubbing, just not sure it would make that loud of a sound since its only felt sound deadening material.

This could explain why you dont hear it when you lift off the gas (the transfer case wouldnt be sending any power to the front diff) and why only some people have it.

If its related to having MPPK exhaust, it could be the opposite side of the transmission where there is the exhaust bracket - somehow the exhaust (or the bracket) might be hitting the heat shield in or around that area. and that would explain again when you lift off the gas, it goes away, since there is little to no exhaust gas going through and not causing vibration. This would also explain why I would hear it coming from the passenger side area - and why people in the passenger side think its coming from the center (they are essentially sitting right on top of it)...

when it gets warmer here I will definitely have to take another look at both of these areas.