I thought I would post the experience I recently had for the benefit of others experiencing the same problem. For the past several weeks my 1981 300SD has been making a knocking or thumping noise when accelerating or decelerating. The noise was originating from the rear of the car. From searching this website and talking with a mechanic friend of mine, I was relatively sure that I had either a bad flex disc, or that the C. V. joints needed to be regreased. I took it to my trusted MB tech, and we went for a little drive.
He listened to my car and determined that the problem was simply a bad hose. There is a fuel hose in the engine compartment that goes from the fuel line to a cylindrical device of some sort. The hose was old and brittle, which was causing the fuel pump to pulsate. The $12 hose was replaced and the knocking stopped. I had some other work done to the car, so my repair total was about $150, but if I had just had the hose replaced it would have cost less than $40. Much cheaper than a flex disc or C. V. joint service. Just thought I would pass it on . . .
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K