My '71 220D had a buzzing noise at speed, about 50mph, that sounded like the exhaust system touching the frame, with power applied. Foot off accelerator and the noise stopped. I spent a lot of time looking at the exhaust system, but it turned out the sub frame rubber bushings were collapsed. Taking out the 4 bolts was not easy, one of the front ones was bent. I was able to get them out, lift the car enough off the sub frame to get the bushings out and replaced, and that did the trick - quiet.

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1971 220D, daily driver, new paint, 142K
1973 220D, low compression
1975 300D, back on the road! 166K
1971 220D, salvage, rear hit, engine excellent
1972 250, bad cam, but runs!
1971 230, engine stuck
1971 220D, low compression, rusty
1976 240D, salvage, engine excellent
1966 230SL, water in oil after rebuild