FYI if it's an unoccupied seat that is creaking also check your tire pressure---I had mine in for service the other week and apparently the porters or whoever put all four of my tires at 38 psi which is overinflated---I just happen to check yesterday.

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2008 X5 4.8
Jet Black/Black Nevada/Dark Burl Walnut
Adaptive Drive
Premium Pack
Tech Pack
Climate Pack
Sport Pack
20" Wheel Option
Premium Sound
Sirius Satellite
Rear DVD Entertainment
Multi-Contour Seats
Comfort Access
3rd Row Seat
Heated Front Seats
Running Boards
3M VentureShield Paint Protection Film