I helped my friend check the valves on his bike yesterday and after we put the bike back together after starting it up we could hear a strange rattling noise coming from the bike. After a short time it stopped, he started it again today and it make the noise again. He rode it and apparently it rides fine but what could that be? It seems to come from the top end and it doesn't sound hard it like a light slapping or rattling. Any ideas? We did end up turning the crankshaft backwards very slightly to line up the punch marks. Could this have caused some problem?