I'm getting an occaisional loud knocking sound coming from somewhere on the bike.( db-30 with 6.5 lifan.) At first I thought it was the motor until I coasted down my street with the engine off. There it was again! The chain is not loose, and I cant think of where else it might be coming from. Everything seems to be tight. The noise is coming from somewhere below me but I cant tell if it's from the front or rear. Any guesses ?