I have a 1981 Kawasaki KZ550, and it started making a noise recently. When I'm at anything other than a low speed, and I let off the throttle or pull the clutch in, I hear a clanking noise coming from the engine. It's hard to explain the actual noise, but it sounds like it is coming from the front of the engine. It keeps doing it until I accelerate again on slow down a good bit. Any idea what it could be?