We have aluminum on our M3 and I had to have several pieces (door arm rests) replaced when we purchased (CPO) I suspected then and was right - it happened again even though we baby the car. For us it's happening every time the seatbelt retracts if you don't hold it while it retracts it will slap against it and ding it. VERY annoying. I also didn't order it on the E70 but turns our its not technically an option anyway unless you're ordering it.

__________________
2010 X5 XDrive 35d (PCD date of 11/2)
Titanium Silver/Black Perf Leather
Sport Activity (ZAP), Premium(ZPP), Cold Weather (ZCW), Rear Climate(ZRC), Active Ventilated Seat (ZAV), Technology(ZTP) & Premium Sound(ZPS) Packages, Adaptive Drive(2VA), Space-Saver Spare(306), Comfort Access(322), Roof Rails (386), Nappa Leather Dashboard & Console (4M5), Automatic High-Beams (5AC), Head-Up Display(610), Satellite Radio (655), iPod/USB Adapter (6FL), Smartphone Integration (6NF)