At low RPM, I hear a rattle or tapping from the top end of the engine. It seems to go away at higher RPMs, but there's a sweet spot from idle to about 1/8 throttle where it's rattling. Nothing is loose, I'm 99% certain it's coming from inside the top end. Cam chain? Tensioner? Valves? Cam shaft? What do you guys think and how can I test to find out what it is. I'm more of a 2 stroke guy, so I'm not sure how to diagnose or fix it, but I'm not afraid to try given good instructions and a manual (which I don't have).