I had the same noise on my 1992 300SE and thought it was rear wheel bearings. Took to an indie and he replaced both. It looked like a big job. Unfortunately the noise, although a slightly different pitch was still there. I pulled the differential and took to a local, oldtime shop which specialized in rearends and with my supplying new bearings and seals and crush washers, the labor to rebuild and setup clearances was $350.00. They said that with 180,000miles it showed no noticeable wear! Put the rearend in and the noise was still there. Back to the indie and he then said that one of the bearings, right rear, had been difficult to install so he offered to replace it again with more care taken this time. That was the problem and now the rearend is silent at all speeds.

Check the bearings carefully as the rearends seem to be well built.

Tim