Have an '82 MB 300D. Started noticing a constant knocking noise from the rear anytime I got it over 35rpm on an incline. Subsided if I pulled over and let it sit for 20mins or so. Now knocking just comes up randomly when accelerating. Goes days without doing it. Eases up when I let off the accelerator. Sound just like engine knocking but at rear of vehicle. Any ideas?
Is this a rear wheel drive with a u-jointed driveshaft? If so, ask your mechanic to put it up on the lift and check the u-joints. That’s what I’d do first if it were my car.
You appear to have a complex drive shaft. There are torsion flex discs at the transmission and the final drive end. These do wear out so inspect them at 30K intervals and replace it/them if damaged. Also you have a universal joint and a support bearing in the middle of the drive shaft. The universal joint could be worn. Also the center bearing may have looseness.
You also have half shafts going from the final drive/differential to each wheel. Check the boots on each of these to see if greese is leaking or contamination has gotten in. Make sure the bolts holding the half shafts into the hubs are torqued to specification. Otherwise, you may need a Mercedes qualified mechanic to inspect the half shafts for wear and/or damage to the joints.
Also check the final drive mounting in the rear. A bushing may be failing or a fastener may be broken or loose.
Hope this helps.