Kayla

84' 928 5 spd stock - motor mounts and torque tube replaced 5 yrs ago, current mileage - 55k miles.

I have a low audible "rumble" with associated low level vibration while driving through all gears and while parked. Motor has a visable vibration (very mild) but not visable in car body.

This is academic since the "problem" occurs both driving and parked but had tires checked for wear and balance - all OK.

Leaves only the engine as the source - any suggestions re: possible source?

ZEUS+

Check AC compressor for loose mounting bolts.

AO

If the vibration is RPM dependent (it changes as you rev the engine) then it's somewhere in the engine or engine mounts. (BTW, if you replaced the engine mounts with Porshce mounts, that may already be bad.)

If the vibration is speed dependent (while driving, put the car in neutral and coast to see if you still feel the vibration) then it's a tire/wheel issue.

Often, tires, and wheels will balance fine, but you can have a bent rim or a bubble on a tire that can give you a vibration. I've found that a "Road-Force Balance" is the only way to determine this. The road-Force balance measure lateral and radial runouts on the wheel.

Mrmerlin

your throw out bearing may be going bad , try a few things one rev the engine with clutch pedal pushed to floor then with it off.
Check the ignition wires for a fouled wire, open the hood in a dark garage look for sparks with the engine idling.
Possible accessory is going bad, remove one V belt at a time and run the engine see if the noise goes away

AO

Just saw the title... "running and parked"