...sounds...dangerous.
I'd suggest turning one pc off, and listen for it, if it's not the first, try the second, if not the second turn them both off and try to track the sound.
Altho i think if it were an actual arching of electricity you'd of known a long time ago. I've had power supplies drop a berring and it can move and make noise, or maybe the fans are dirty and kind of lerching. Power supplies are pretty inexpensive nowadays, i wouldn't hesitate to replace them...not only for your pc's sake, but for the safety of your house :|