Vibration at all times stating stopping idling

92 nissan stanza 165000 miles ok dad has eliminated the following not drive shaft not harmonic balancer, changed out the tranny, might might be torque convertor but he changed that too. Not, the cv axles, changed timing belt this is not a vibration like a missfire or anything he is very insistant about that,
when car is running it vibrates like a vibrating chair it is driveable but still vibrates and you cant hear over it. So he has never seen anything like it and is stumped can anyone help?