I can tell you from experience that you should be checking the Clearance on that Mail Bearing... Put a Dial-Indicator the crankshaft and lift on it.. Mine had .073" Clearance and didn't make any noise at all..the only problem seemed to be that the seal was letting engine oil into the transmission.. Had to replace that Main bearing AND Rod bearing..all because of the "Gasket" that is trapped between the engine crankcase and the bearing quill. That "Gasket" had swollen and blocked the oil flow to that side of the crankcase. If you do install a Bronze Main Bearing, have at least .005" bearing clearance..I used .004" and had a hard time breaking it in. Let me know what you find, but I am SURE this is your problem..