I have the same thing on my wife's PC (server PC) using onboard VIA "Vinyl" AC'97 audio.
It appears to be electronic interferance from the motherboard components and the internal wiring.
IE., "improper wiring" is the answer, but it's nothing you can do anything about - it's improper *motherboard* wiring. Too many high-RF-generating components close together.
Anyway, the easiest way to minimize it is to crank your speakers down as far as possible. What you want to do is go into the Windows "Sound and Audio" control panel, hit the "Advanced" button and crank the "Wave" volume ALL the way up, and turn up the main volume slider (leftmost) up about 80% or 90%.
Keeping in mind this should be "loud", turn your speakers down until this is just about the upper limit of 'normal' volume. Then, use the Windows volume control to control loudness rather than your speaker's volume control.