I have the same type of noise, 09 328xi automatic. It's what a call a grinding / humming noise but it also has a thumping noise only when you let off the gas, most noticeable between 40-60 mph, kind of sounds like a out of round tire noise but goes away as soon as you step on the gas. I took it to the dealer and they "assume" it's low on fluid due to an accident the car was in 30k ago! I just bought the car, first BMW for me so I'm not sure how to proceed. I have known the car since new, i know the damage that was done in the accident and it was fixed 30k ago so its hard to accept this problem is due to the accident. Any ideas for me? If it's just low on fluid where to I go to check it? I'm a do it myself kinda guy and I can turn a wrench so any suggestions would help.