I am hearing anoise like something is shaking or loose and you can only hear it really when the truck is idle and when I start my full size blazer. When I look at the belt I can see the bottom pulley shaking like it is lose. Is that normal? Since it is shaking could that cause the knocking? Does anybody know the name of that pulley?

 

The bottom pulley is attached to the harmonic balancer and that is bolted to the crankshaft. It could be possible that the balancer has become out of balance and cause the shaking. It's hard for me to explain how to check to see if the balancer needs to be replaced. There is a strip of rubber within the balancer, you could see if the rubber material has deteriorated or the outer ring has shifted.

 

Should you just be able to look down at the crankshaft pulley and see it shaking while all the other pulley's dont shake at all? Does anyone know about how much a crankshaft and a harmonic balancer cost?