I had this problem about a year ago. X was only six months ol and I was sure it was something more serious. Turns out they put some type of film around the upper door openings on the body of the car where the felt of the door frame rubs and it completely went away. It's been over a year trouble-free.

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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)