Ok, now that we know its been rebuilt this changes a few things. Did he rebuild it himself or was it professionally done? At first I was thinking he may just be hearing spark knock/light detonation. But if its a hollow knock for sure it could be a more serious problem. I doubt however that its "piston slap" because he should hear that all the time. It comes from a broken piston skirt. It could be a wrist pin but he should also hear that more than just when he gets on it hard. If he can verify that it is only a problem when he is running it hard then I doubt its anything in the shortblock. He may just have a loose follower. How long ago was it rebuilt? How many miles since the rebuild?