I had the same problem, it turned out to be the belt tensioner pulley. This was on a 190E with 115K. When the belt was not on the pulley, it ran smooth, but with the belt on and the engine runnung, it growled quite a bit.
83 W123 240D
76 300D, "Lil Yellow Car"
1978 Cessna 172N 180 HP Mod
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