Wheel bearing possibly?? I had my X5 in 3 or 4 times, the dealer did not want to change the wheel bearing because they were not sure that is what it was...I brought it to a different dealership, and we took a chance by changing it, and it went away (we changed rear passenger wb)!!! that was a happy day, as the droning between 70 and 85 miles/hr really wore on me...