No, your amp is not clipping. I assume you mean the "popping" occurs when there is a loud sound effect in the program material such as a thunder clap or other loud bang.

Your center speaker is most likely bottoming out. The cone moves back and forth in a linear travel. The pop you are hearing is the voice coil (card board tube wrapped in fine copper wire) hitting the back plate of the speaker motor (magnet). That pop means you are driving the speaker with enough power and enought low frequency bass signal to exceed its physical limitations and hit that back plate. In otherwords, your going "balls deep" with your center. (How's that for a mental image?) It can cause damage to the speaker. This is why many subwoofers have a "dome" backplate or no backplate at all.

Possible solutions: Turn down the volume, turn down the bass, set your center speaker to "small" thereby limiting bass response, if your reciever has an equalizer for each channel then you can knock -6db from 63hz (i.e. again turning down the bass), or get a center speaker with more bass capability.