my mechanic says it's a piston w/build up on it that needs to be re-seated.
We has a Ad on TV...mechanic holding a spark plug " we had to remove the sump, then pull the crank and piston, found dirty spark plugs"
"A piston with build up " IF that is true means carbon build up...carbon build up doest cause tapping, very unlikely with modern engine maintaince computers...and simply by pporing a small steam of water into the engine while running will re carb it
And "re seat it" re seat a pistion???
1st simply do what I suggest in my post above yours...locate the tap.