Strange noises wenh i use my computer

intel i7 920
asus p6t
12 gb ocz ram @ 1600 mhz
Thermalright ultra 120 heatsink
nvidia gts 250
ultra lsp650 psu
nzxt tempest case w/ 6 fans
cyber acoustics speakers.

Okay that's my specs, here's the problem...

My computer speakers often make strange noises especially right after boot up, when scrolling with the mouse on the web, and even while idling.

The sounds are a mixture of crackling and squealing.

I am using a wireless mouse and keyboard and I am sure that it is not caused by my cellphone.

The problem seems to disappear when I play audio, or at least the sound is drowned out to the point of inaudibility.

One big problem with determining the problem is that the problem is somewhat intermittent and doesn't happen all the time.

Any suggestions would be very appreciated. Thanks in advance!

also, I don't have a sound card...