On my bike it was the big nut you removed to get the flywheel off....that was loose.  That big nut was finger tight when I removed it.  the noise on my bike came from the crankshaft/flywheel banging on those splices every time the engine fired.

Pull up/down on the end of the crankshaft, and see if there is any play or movement.

Was the big nut really tight when you took it off?

How could you go wrong with a professional mechanic?
They replace head gasket, well, they Charge for it. Probably just fix the plug..
This bunch,, got the professional bunch Beat.