Occasional Thunk Under Back Seat/Trunk

Occasional Thunk Under Back Seat/Trunk

I regularly hear thunks coming from under the car somewhere toward the driver side/middle under the back seat/trunk. This seems to be particularly (but not only) likely to occur about 60% of the way up a moderate incline hill. Is anyone else having this issue? Any idea what it is? Thanks!

Iceniner | December 5, 2018

I have an AWD M3 with about 2,500 miles but this has been happening all along.

Magic 8 Ball | December 5, 2018

It is a know issue that is most likely due to thermal expansion. If you get in the back seat and push down with your foot on floor, particularly behind the driver seat (my experience), do you hear a hollow "drum" like sound similar to what you are hearing?

nikhilm_2000 | December 5, 2018

I have a dual motor TM3 and have been experiencing it as well, mainly going uphill. I have a service appointment to look at that along with some other things in a week so will report back as to what they say.

Bighorn | December 5, 2018

Every couple days, someone starts this thread.

Iceniner | December 5, 2018

nikhilm_2000 That would be great. Thanks!

82bert | December 5, 2018

@bighorn. Agree. This is normal folks. Don’t waste your time going to service.

fuwil | December 5, 2018

I always hear it soon after supercharging starts. Given superchanging generates lots of heat, it’s very likely it’s indeed due to thermal expansion.

billlake2000 | December 5, 2018

I don't know what a thunk is. Can you post a picture please?

mntlvr23 | December 6, 2018

I have that dull noise emanating from my trunk as well - but it is just my mother-in-law

billlake2000 | December 6, 2018

mntlvr23, in your case, picture is not enough. Need video WITH sound.

mntlvr23 | December 6, 2018

@billlake2000 - oops, she stopped making sounds last night - not good - so much more road noise coming through now