I would highly suspect the mounts and theres a way to check. If you have someone with a similar car with good engine mounts raise the hood, grab the intake manifold or whatever you can grab onto and try shaking the engine side to side or up and down (don't hurt your back trying). It should hardly budge in any direction.
Then do the same on your engine. A mechanic would use a floor jack under the engine to do the same test.
Greasybenz was just by the other day, complaining his fan was hitting the shroud. Well, I could put one and a half fingers between the fan blades and the shroud, his suspicion was the shroud was too low or too high or something.
So I did the "engine shake" test first on my '87 300D with new engine mounts then his. You could see that his engine was moving considerably, it was obvious the mounts were the problem and looking at L mount you could see it was coming apart.
I suspect bad mounts also put a lot of stress on the front flex disk so better check that also.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!