My '83 300D turbo, with berely 117k miles on it, makes a lot of noise, more than the usual dieseling, but only at specific engine speeds. When cruising at an indicated 65-75mph there is an odd, almost groaning sound from the engine. As soon as I accelerate to 78-80mph the sound totally disapears. Similar sound develops at lower speeds but the same RPM if I hold it in 3rd or 2nd gear.
The car is in superb shape otherwise. I have changed the engine+trans mounts and examined the engine shock/mounts, all seem ok. Everything on the engine seems well tied town, including the troublesome air cleaner assembly.
I have been told that a worn turbo might make such a sound. Does anyone have similar problems?