Chevrolet Repair: Bad Vibration: 1979 Chevy C10 Bonanza, chevy c10, bad hope

Question
I have a 1979 Chevy C10 Bonanza P/U. I bought it used and the engine has been either rebuilt or replaced. It has a 350 V8 with Auto Trans. The problem I have is a bad and noisy vibration, mostly at idle. It nearly rattles out my fillings. One mechanic told me it was probably the engine mounts, said they were worn out. I had them replaced but that did not resolve the problem. Any ideas what this could be?

Answer
Hi Laura.... it could be a couple of differant things.... check the harmonic balancer, it is located on the front bottom of the engin attached to the crankshaft...it could be out of balance...or the wrong one for that engine. you can replace with a fluid balancer. if you look at it with the engine running you should be able to see if it is out of wack... it should run smooth...if it doesnt replace it...also you could have on the wrong flywheel, they are balanced as well...if the problem is mostly at an idle i would lean towards the harmonic being bad.... hope this helps