[arkansas]

Someone else mentioned the harmonic balancer. I have a 400 in an ratty 49 coupe that had something similar going on with. The key in the crank had somehow gotten wallowed out and would let the balancer turn some like you mention. The knock I had wasn't too deep, but was the balancer turning back and forth hammering on the key which had lain over some in the keyway. It might save you from dropping the pan to check that first. On mine I pulled the balancer and timing cover, stood the key up and welded up the wallowed out area. Dressed it with a dremel tool and was up and running again. So far it has held.