The fact that the noise drops off at higher speeds is telling. (If it were "road noise" the intensity would continue to increase.)
So it seems logical to conclude that there's something funky with either the transmission or possibly the differential.
It would help if you could better pinpoint from where the noise emanates (I've found that opening the windows--reducing the echo chamber effect--and having a friend listen from different locations in and around the car sometimes faciltates this.)
Good luck in solving what I'm sure is a frustrating problem.
1991 560SEL-Euro (145,000 miles)
1989 420SEL (Retired at 325,000 miles)