Why is my transmission popping when I press the gas?

Not sure what vehicle you have, and if the tranny does otherwise operate normal, it could be the mounts. Make sure they are not loose or broken also check the crossmember mounts if your vehicles is RWD. Also check the motor mounts to make sure that the noise and motion does not just appear to come from the tranny. Now granted, all these are just guesses since it is hard to determine what noise it is and what else is going on with it, but it should give you a place to start looking.