95 Riviera engine noise

I have a 95 riviera 3.8 sc with 67,000 miles, recently I was driving and heard what I thought was a rattle or noise. You hear it when it starts, shuts off, idling when in gear, but stops or is very faint when you give it gas in gear or not. Pulleys are all good except the crank. It makes a half turn in each direction freely, then stops with the belts off. Car runs and drives great, no loss of performance, just this noise. I have a parts motor and the crank does not do that,what do I need to replace?

Your harmonic balancer is shot...rubber has torn loose between pulley and hub- classic symptoms are a knocking noise on start-up and shut-down- just as you describe. You have diagnosed correctly, now change it out- takes a puller on later model engines, as is yours. Not a bad job. In fact you can practice, using your donor engine first.

Hardest thing is breaking the crankshaft bolt loose- I have used a breakerbar mounted against the frame rail and used the car's starter to break it loose. Works even when an impact gun won't.

Your harmonic balancer is shot...rubber has torn loose between pulley and hub- classic symptoms are a knocking noise on start-up and shut-down- just as you describe. You have diagnosed correctly, now change it out- takes a puller on later model engines, as is yours. Not a bad job. In fact you can practice, using your donor engine first.

Hardest thing is breaking the crankshaft bolt loose- I have used a breakerbar mounted against the frame rail and used the car's starter to break it loose. Works even when an impact gun won't.

Thanks for the advice sure did fix it. Hardest part was replacing the seal, getting it to go in straight without damaging. But it worked only costing $25. 22 for the seal and 3 for the bolts for the puller. Thank you again and hope you can answer any problems I have in the future, hopefully none.