scary huh?

I've got this on my pc soundsystem too...

my pc soundcard haves a feature called eax, this is an upmixer for stereo to surround, but when I play a surround game, and I leave the eax feature on, it also creates the popping, cracking sound, when it isn't gone then, I have to turn the volume down in the game (in options eh, not the actual volume of the speakers)

I think your problem is similar and that it is caused by overpowering the signal for the center channel input on your receiver, causing it to 'clip' (I think)

so, some things you could try: disable all upmixing effects, especially the ones that make use of the center channel alot, try to get a discrete signal, and when possible, turn down the volume of the center channel, only the center channel...