I had a similare problem with my 1988 560SL. The noise eminated within the car from the shifter area. While under the car wheels up and my wife running the car at ~ 60 mph the noise appeared to be comming from the tail end of the trainsmission. However the center bearing is covered and I belive the noise ends up comming up the drive shaft and throught the opening at the tail of the trainsmission. If you do a search you will see my post on marble noise's from the transmission. I ended up replacing 5 parts.

1) Transmission output shaft bearing
2) Center bearing support
3) Center bearing support bearing
4) U-joint
5) Driveshaft extension dust boot

I cant say for sure but I belive the real problem was the drive shaft extension dust boot. It was torn and had a piece of rubber swinging around and possibly hitting something. The other parts were replaced only because I had them out . The U-joint was only slightly stiff and the center bearing support was sagging. I dont belive these had anything to do with it.

Good luck
John Roncallo