My '79 300SD, makes a noise similiar to the subject of this thread at 37 MPH. It also makes it under load uphill. The noise comes from the rear end. I have no noise at all at different speeds, or going downhill.
I have not ventured under the car yet, due to the record setting snow cover in these parts. I wonder if this is a typical flex disc, or center bearing noise, or maybe tranny mounts?
Thanks to all who reply!