If you are hearing something striking the piston, it can only be one of two things, an intake valve or an exhaust valve (providing rings are still seated). Everything goes away at higher rpm's,
. If you hear a distinct pop in a cylinder chamber, with the car off- rotate the crank by hand with a socket and rachet slowly and listen for the pop noise on a compression stroke. If you hear it when rotating the crank by hand you will be able to identify which cylinder it is in and more importantly its exact point of origin and contact. While a piston striking a slightly bent valve will making a distinct popping noise and the car will run, it is not good, so eliminate that, sticking valves in zero tolerance engines can be bad news, especially if the piston has already struck the valve and bent it slightly. I hope I am wrong. I have seen where a door screen screw got into a cylinder from the spark plug galley and rode the top of the piston popping as the piston moved up and down, that was the weirdest thing I think I have ever seen happen as far as 'popping' goes.