I thought it was this too, but I could stop the rattle by pushing on any part of the rear panel. I removed the panel and found a plastic piece attached to the speaker wires that was not set in it's correct spot and was banging against plastic. Easy fix.
2009 e92 M3 Individual Laguna Seca Blue
|Black Novillo|DCT|19"rims|Carbon Leather|Premium|Tech.|Heated Seats|Prem. Sound|iPod|
Mods: |Electronic Performance Steering Wheel|ZCP CF Spoiler|ZCP CF Spliters|ZCP CF Mirrors|Edition Front Grilles|Edition Side Grilles|BMW Performance Brake Handle|3D Design Pedals| Style Rumore Exhaust|H&R Springs|LCI Tails|Challenge CF Door Panels|