Fred and I have been collaborating on this problem today since I have the EXACT same problem. Both of our cars are '92's and have the same mileage (approx 89,000). Mine is a 500E and Fred has a 500SEL. The problem is ONLY in reverse. It is HIGHLY pronounced as well and seems to be getting louder with time. I noted to Fred that when backing up, if I turn the wheels sharply, the sound might cease and start again when the wheels are straighted out. Also, if there is an elevation change like when the driveway meets the street, the sound might cease. Maybe it is something in the suspension... bearings? I have two questions:
What suspension parts (major) are the same between these two cars? AND What suspension parts are ONLY engaged in reverse?