Hello and welcome to TM and the many issues of the M3W. I a friend of mine have the same issue with our M3W as you have with the under dash. Their is a plastic bit that is just over the top of the instruments, I believe it is there to keep water out of the wiring and instruments. It can make all kinds of racket when hitting a bump in the road. My firend stuffed insulation a top of this bit and fix his issue. Not wanting to have fine fiberglass in my car, I'm going to get some anti-vibration stick on strips from a hardware store to stick on the top of it to fix it. You also can find it at a good car stereo place that does installations.
