Whats up I have a question about piston slap

ok i just got my motor rebuilt an at idle i hear a slap noise the guy that built it said it was piston slap so when im at a light i hear the slapping noise but when i hit the gas i dont hear it anymore an it still makes the sound when the car is warmed up so what does anyone think about this nosie the guy that built it is a honda tech not no back yard builder