Electrical Problem

I put together a pretty nice H/T system for my parents some time ago. Recently, a loud, sharp, popping or cracking sound has begun emanating from all speakers (except possibly not from the center channel. I can't verify that, though). It occurs at seemingly random intervals (not necessarily when there is a heavy load on the amp and I can't tie it in with A/C switching on or other household appliance) and varies in sound level. I haven't added or changed any components or wiring lately, and it all seems to be in good working order. The system is plugged into two Panamax surge/conditioing banks.

Any ideas from where this noise is occurring? It is almost loud enough to damage the speakers and has me worried.

Thanks for any and all help!!!!

Brad Day
Atlanta, GA