A few of us have discovered the source of the problem is actually from the black plastic at the bottom (side) of the driver's seat. If you shift it around a bit (it does move), you should be able to get it to stop. To test, you can shift your weight around in the seat beforehand and see if you can replicate the sound.

The door seems solidly built so creaks are less likely from there.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr