Detonation... Your fuel is getting ignited sometimes at incorrect times, maybe due to overcompression, or a too-hot plug. Probably knocks the piss out of your whole motor when you hear it. Try a colder plug maybe, or raise fuel octane so it doesn't ignite till lit properly.
I had that happen and I got a (very cold) B9 plug instead of the B6 that was in there, and it stopped. Try going only one number colder at a time till one works right, if carboning occurs, go a bit hotter again. Find the happy.
If hi grade gas works, it will and you'll notice, if it seems to have no effect, it _is_ having no effect, and is unnecessary. Premium isn't better gas, it is gas that is a little harder to ignite (and actually has a little less power potential, not more).
Check for piston clearance in the top end. Make sure that the piston isn't slapping up on anything in the head...like the head...