A friend is bringing over a late model A for me to check out. It has a loud knock every time it fires. I have done the pry bar to the ear listening and can't figure out where the knock is coming from. I have listened on both sides of the crankcase, block and governor. No matter where I check, the knock sounds the same. I checked the clutch driver and it is tight on the crankshaft. They said they checked the flywheel and it's tight. I had them put it in gear with the clutch released and that changed nothing so I know its not in gearing. Any ideas before I start tearing things apart? Thanks.