Hello. I have recently purchased a 1982 300D Turbo sedan. I am currently chasing down various quirks in my car and am currently stumped. I have a low level hummm while driving at sustained speeds of 50 mph or more. This noise remains as long as the engine is supplying any power. When the car is put in neutral and coasted the sound goes away. If I just release the accelerator and produce a mild deceleration the noise goes away. If I drive at 60 to 65 mph for a sustained time, 45 to 60 minutes, the drone is there but not objectionable. When I pick up the pace to 70 to 75 mph the noise goes to a mild rumble after about 40 minutes. I once noticed a slight vibration when the rumble was present at high speed. I started chasing this down and have found the following. After aproximately 15 minutes of driving the differential is warm, not hot just warm. The drive axels never seem to heat up to indicate a worm bearing surface. I have never noticed a car with a differential that warms up. Also in checking backlash at various places there is no lash from the wheels all the way to the transmission. The only lash is in the transmission. I am concerned by the fact that there is no lash from the tranny back; I have never had a car with no lash. Again I have never had a rear end that did not have backlash. After driving the mid drive shaft bearing is not heated up so I do not suspect this part. I have raised the car and the rear wheels rotate without any restriction and there is no rock side to side. When a sideways force is placed on the wheels and they are rotated there is also no restriction or noise. I have checked my Haynes manual and cannot find any information on the sleeve nut on the drive shaft. If this is not adjusted correctly can it cause a restrictive length to the drive shaft assembly? I have drained the diffy and found the fluid to be up and it was dark but had no filings or any other debris in the oil. The boots, both outboard and inboard, are intact and not leaking any fluid. Now I am stumped. I would rather not disassemble the entire drive train till I find a culprit.

Can someone please offer advice? Oh, driving under 60 mph and replacing the car are not options. :-) Sorry for the post being so long.

Thank you in advance for your help.

Rick