I have a mid RPM engine vibration in my 76 300D that has me completely baffled. It is like a harmonic vibration that happens going up to speed and then back down through the RPM range coming to a stop. I have read every post I could find for clues. This is what I have done so far.

Replaced engine mounts with new original MB parts.
Replaced tranny mount.
Replaced fan clutch (old one had a lot of play in it)

When it hits that certain RPM range the vibration can be heard throughout the car and felt in the gear shift lever. When sitting still the vibration is still there when you rev the engine which tells me it is not in the drive line or transmission.

Any clues to what I am missing?
Thanks for your help.

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Western NC
1987 300SDL W126 Turbo Diesel 335,000 + miles
1976 300D W115 275,000 miles+