I don't know if this is a possibility with your engine, but it came to mind. Maybe some of the others can tell you if this is possible with your engine or not.
Years ago, the mechanic in the stall next to me had a noisy situation after a top end overhaul. Not the same type of engine but I suppose it could happen...
He used the head gasket from a smaller motor with the same head bolt pattern. The piston was slapping the steel ring of the gasket that was smaller than the cylinder. Made a noise we were unfamiliar with and we checked a lot of things before tearing the head off and finding out the answer.
