Most definitely rear axle whine. Especially if you let up or get on the gas and it disappears. Thats the way mine is anyway. From about 60 mph to 80 mph. I was reading a thread on the MB boards from a guy that had this same issue and the MB tech said yep, thats not right and ordered up a new axle assembly and the noise was gone. Do think our five fingers dealers would do that for us. I think we need to see how many actually have this issue. I think it is alot.