Somehow my engine (and airplane) hit the ground inverted wide open. After cleaning it out (removed everything except the crankshaft) and re-assembling it runs fine except it make a loud grinding metal to metal sound (like the crankshaft is rubbing against the rear plate). I've ran it about six tanks now and it is not getting any better. I've inspected the back plate and do not see any wear..
The engine seems to be running fine. It just sounds bad.
BTW: there is no slop or play in the crankshaft side to side or front to back.

Should I be concerned or just run the damn thing and not worry about it?

FWIW: it's an OS 61FX