Windows audio? Do you mean like media player? I am not sure if this is the case here, but I used to have a G4 tower in my studio, when the wireless card was enabled and I was playing back audio through iTunes to my Digi002, there was some kind of strange sound that would come through the speakers every time a network packet was sent (accessing web pages, or in the background clock sync). Disabiling the card solved the problem. Obviously, my G5 doesn't have internet access on it now. Strictly a Pro Tools machine. Perhaps looking into disabling the network card could help. Also, is it a clipping noise? or just cracks? Perhaps your monitors are too hot?