I have mentioned this as a response to another post but that was many many months ago and I was hoping people have new ideas or even solved their problem...

What would cause a vibration in the seats (not steering wheel) when accelerating or even coasting down between 55-70mph? Anything higher or lower and it's smooth again. I have had them balance 2 times (there are weights on the rims) and did a wheel alignment (to make sure) as well... problem remains, on different roads so I know it's not just a stretch of highway, etc. This does not occur at any other vehicle speed. Number of passengers doesn't matter either. I can actually see the seat vibrate too.

From the old thread it looks like I'm not alone on this issue. I have replica wheels which were spun to ensure they were not out-of-round on Toyo Proxes S/T. They are at 36psi (would different tire pressures cause this at certain speeds only?)... I ask because this used to happen at a different range (70-75mph only). The stock 19s never had this problem.

**EDIT 11/4: see resolution below.

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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