[Medic-NY]

So a few years ago, I bought the H&R Sport springs for my F34, had them installed professionally, and the ride height was perfect! Handled so much better than stock. No more rear end float.

However, if hitting any bumps or pot holes, there is a very loud bang under the wheel area, similar to if you hit your rim really hard in a pot hole.

I have checked and tried to compress the suspension to see what hits, and there is NO component interference. Nothing rubs or hits, not even close. The sound seems to come from high up under the hood, so all I could think of was the strut is bottoming out??

Anyone have any other ideas?

[Call me the breeze]

You didn't mention if the noise started at the beginning of the new springs or got progressively worse over time. Also no mention of changing out the struts and shocks.

Assuming that have the factory struts and shocks and the noise has worsened over time the likely cause is the front struts bottoming out and hitting the bump stops at the strut tower. Mismatching spring and strut/shock heights will destroy the struts/shocks. They are designed to have the same neutral point. If the spring is shorter, it will cause the strut/shock to always be in compression and eventually fail. My recommendation is to replace the struts/shocks with the shorter Bilstein B8s which pair nicely with the H&Rs. There are lots of people who swear by this combo. This will be a stiff setup so potholes won't be nice but cornering will be on rails. Even if my theory is wrong, I wouldn't wait until things get worse. Your setup is not particularly safe. Also the strut towers could develop cracks. This was a known problem in older E46s. They are not designed to take impact forces.

[Medic-NY]

Originally Posted by Call me the breeze

You didn't mention if the noise started at the beginning of the new springs or got progressively worse over time. Also no mention of changing out the struts and shocks.

Assuming that have the factory struts and shocks and the noise has worsened over time the likely cause is the front struts bottoming out and hitting the bump stops at the strut tower. Mismatching spring and strut/shock heights will destroy the struts/shocks. They are designed to have the same neutral point. If the spring is shorter, it will cause the strut/shock to always be in compression and eventually fail. My recommendation is to replace the struts/shocks with the shorter Bilstein B8s which pair nicely with the H&Rs. There are lots of people who swear by this combo. This will be a stiff setup so potholes won't be nice but cornering will be on rails. Even if my theory is wrong, I wouldn't wait until things get worse. Your setup is not particularly safe. Also the strut towers could develop cracks. This was a known problem in older E46s. They are not designed to take impact forces.

It was like that pretty much from the beginning. I kept the stock OEM struts, and only replace the springs. They are not on my car right now because the banging was so loud it literally felt like the car was tearing itself apart. Also, I took the bump stop out and tried it that way as well and it still made the noise, so I'm assuming the struts are bottoming all the way to the strut plate, because I can't for the life of me find anything else that hits or interferes. Also thank you for the B8 recommendation, I was looking at getting those and hopefully that would solve my issue. I'm going to just go ahead and get them.

[Medic-NY]

Originally Posted by Call me the breeze

You didn't mention if the noise started at the beginning of the new springs or got progressively worse over time. Also no mention of changing out the struts and shocks.

Assuming that have the factory struts and shocks and the noise has worsened over time the likely cause is the front struts bottoming out and hitting the bump stops at the strut tower. Mismatching spring and strut/shock heights will destroy the struts/shocks. They are designed to have the same neutral point. If the spring is shorter, it will cause the strut/shock to always be in compression and eventually fail. My recommendation is to replace the struts/shocks with the shorter Bilstein B8s which pair nicely with the H&Rs. There are lots of people who swear by this combo. This will be a stiff setup so potholes won't be nice but cornering will be on rails. Even if my theory is wrong, I wouldn't wait until things get worse. Your setup is not particularly safe. Also the strut towers could develop cracks. This was a known problem in older E46s. They are not designed to take impact forces.

Do you know if I need to retain the factory bump stops and dirt guards for the B-8 as well?
If so I will just get a whole new set.

[Call me the breeze]

Hopefully you won't ever hit the bumps stops again with the B8s but they are not an expensive part to replace. I am running the B6s (stock height) with factory springs. They got rid of the floatiness and braking nose dive. There's a YouTube video of a guy comparing Bilstein to factory strut. He could literally compress the factory strut with one hand. He could barely move the Bilstein.

I also own a 2004 330Ci coupe. With the original suspension it handles better than my upgraded GT suspension. I wasn't expecting the GT to ride on rails but the engineers have de-tuned their products to the point that I'm not sure they still deserve to be called the Ultimate Driving Machine. Sounds like your quest for better handling has landed you in a similar place.