The E90 has HVAC vents under the front seats with holes big enough for change and stuff to fall into it. So when you stop, it slides forward. When you go, it slides back. Maybe it rattles a bit on start-up. Not sure about the E92, but is it possible there's something like a marble or change in the vent?
330i | Black Sapphire | Beige + Burl | Sport | Premium | Cold | Nav | CA | Sat prep