Check the flex disc. Mine did the same thing and the disc were falling apart.

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Von
Fairport, NY
1973 Unimog 416 Doka
1980 Unimog 416 Doka
1981 Unimog 416 Doka
1984 Euro 280CE w/diesel conversion
1985 300TD Estate wagon
(I really need to stop buying these things!)
http://vonsmog.com