It's probably pinging (detonation)
It happens when you're running too lean, too high of a compression (add shims like mentioned before) or too high of a nitro level.
What happens is the engine heats up so much and it ignites the mixture on the upward stroke instead of the downward, causing damage

Here is my head from a OS CVR which has detonation marks on it, the piston head has similar marks as well