Vibration on start-up
 

I have a 1982 911sc that has a strange vibration on start-up. It occurs after it has sat for more than a few minutes. It sounds like something out of balance and last about 15-30 seconds, depending upon how long it was since I last drove it. In the past the sound has gone away when I depressed the clutch and engaged any gear. The car has a rebuilt engine with 179,000 miles and nothing has been done to the transmission. Nothing is loose in the engine compartment.

Any ideas?