I have this problem both on my desktop (nforce2) and laptop (AC97). I'm
getting this extremely irritating noise on my audio whenever there isn't any
sound! I can be listening to something with Winamp, be playing a game, or
anything that creates noise, but 3-5 seconds after I stop my speakers start
to make athe noise. Disk and network I/O will alter the noise (which sounds
like a high-pitched note and constant beeps with changing frequency), but the
only thing that stops it is if I start up music (even if the volume is at 0%).

Muting individual channels, master mute, volume changes - nothing of this
nature affects it. I do have up-to-date drivers for everything and I've
disabled all things that I don't use in the bios. I was first suspecting my
802.11 cards, but removing these has no effect. Please help! This is
driving me nuts! I'm really perplexed on why I'm having this same problem on
two very different machines. Oh, and the problem does not happen in
Linux/BSD.