What's snapping?

I have been getting this "snapping" sound through all speakers from time to time and last night it was driving me crazy. It was happening every couple of minutes. It sounds like a static charge releasing itself, if that makes any sence. It does it on all sources. Right now I am using a Harmon Kardon AVR7200 as a preamp for a Mcintosh MC352. It was doing it before the Mcintosh arrived so that can be ruled out. I think it must be the HK or the Monster 2100 powerbar. Is it possible it could be a bad cable or dirty connection even though it happens on all sources?
I am finding it difficult to narrow down because it doesn't do it all the time. Today the system has been on for almost 2 hours and it is fine.
Any ideas ???

Thanks