87 240 stick-shift w/ 201K miles. Just started driving it. Has a vibration - almost a buzz - that you can feel on the shifter, dash, door, etc. Almost feels like the kind of vibration you get if the exhaust is hitting the frame somewhere.

It increases w/ motor RPM and goes away at idle and coasting. It's also bad - maybe worse - when the engine is lugged, say if you shift 3rd to 5th at low speed or slow on a hill without downshifting.

Parked and revving the motor there seems to be no vibration. Touching the motor by hand or w/ probes - nothing.

Carrier bearing and mount, engine mounts, trans mount, u-joints, bushings - all good. But shouldn't be one of these as it's RPM - not road speed - related. Exhaust rubbers are good.

I suspect something out of balance on the engine: crank pulley, flywheel, clutch. But any of these should be noticeable when revving the parked car. Mainshaft in the trans? bad engine main bearing?

Any ideas and/ or anyone seen this kind of problem?


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240s: 2 drivers and some parts cars