I have a 2011 R56 Just a that had a back end rattle from the drivers side and it was driving me insane! I tried everything I could find on the forum to solve it with no luck but I finally found it on the weekend. The culprit was a $2 plastic expansion screw that has mysteriously gone missing from the wheel well. It's the one on the back that holds the wheel well trim to the rear bumper skin through a tab so on every bump the bumper was rattling against the tab. Quick trip to the dealer, $2 and a stubby screw driver and my rattle is finally gone and I can now sleep at night again!