Might be the graphic-card. AFAIR the capacitors on it sometimes make noise. Mine is clearly audible when i run glxgears (I'm running Linux on my PC). It's a kind of constant electronic, very fast chirping sound like it's coming from a very small bird on steroids.
Another strange experience I had was that the speed of music-playback was slightly changed, constantly going up and down a bit, resulting in a slight de-tuning when downloading while playing music. During my research I noticed that the onboard ethernet-controller and my soundcard shared the same IRQ. After changing that this behaviour vanished.
You're not alone with this! The white dressed men are already coming for you!
