I have it and had complained of the sound early on but I haven't heard it for a long while now. Dealer techs say it's normal flex from the vehicle which is really rigid to start with so that's what happens. But I remember reading an explanation from a member here that it is a reflected sound from a pan on the undercarriage. My memory is horrible - hopefully I can find the thread.

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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