I have a 1995 BMW 540i, with about 260000km, and it has a real mysterious vibration that starts up at about 80-90km/h (~50-55 mph). As I speed up it gets even stronger and hits a peak at about 140km/h (~85-90 mph), but it's there all the time. It does not seem to be coming up directly through the steering wheel, although this is vibrating too. Rather, the whole car tends to shake, and all the seats, dash, ... vibrate and jiggle around. It's not terribly bad, but pretty uncomfortable for an autobahn car.

Ok, here is the long story: The vibration was there a while ago, I took it to a shop, they checked my wheels, tires, suspension and steering links, bushings and driveshaft mounts. Eventually they said the rear tires were slightly flat spotted (but not enough to cause a big vibration) and rear brake rotors were warped, replaced these, vibration subsided but still started at about 140km/h. I did drive on some fairly bad roads since then and finally it came back altogether and now it starts at 80 again. I live in Toronto and I took the car to Bruno at Racing King to check it out. He checked all the suspension and steering links again, wheel bearings, motor and transmission mounts. He told me the thrust arms bushings are slightly worn, as is one of the steering links, but none serious enough to cause such a big vibration.

So it is all still up in the air. If anyone has experienced anything like this and knows what it might be, please please let me know! it's driving me nuts!