It's some sort of signal interference (or white noise) on the sound card. I have the same problem on mine when I use headphones connected directly to the sound card output at high volume
twisted: Sounds like a 747 winding up for take off while any of my optical drives spin.), but it isn't there if I use my keyboard audio controls to pull down the volume...
Possibly easy solution; pull down the device volume of your sound card in your control panel (under Sound and Audio Devices) and boost the volume on your speaker system (if your using ampified speakers). That should make it go away. I noticed on my machine that the problem became exponentially worse as I increased the output level.
Fyi, I recently upgraded from onboard Realtek AC97 to a dirt cheap Creative 24bit Live! and noticed a significant reduction in white noise at full volume using heaphones compared to the onboard audio. So if your using an onboard audio a cheap upgrade (I spent about US$40 on my new audio card) might also help.