piston knock.......

Could be from worn wrist pin bearing, or wear in the cylinder. A top end rebuild would more than likely take care of it.
 
it could also be your main bearing's carbon build up or bent conrod but its hard to say if a person cant see\hear it for them selves
 
probly a worn top end, do a compression test. that will tell you if you need to do one.

i thought for sure somebody was gonna bring up detonation. which may cause a noise too.

blaster engines are a bit noisey, but idk how your sounds, so its kind of hard to tell.
 
i know if u have a leak in either one of the top end gaskets it leaks and knocks freakin hard...you would notice the oil/carbon build up on the bottem end cases though...but it does make a hard knock when they go