Weird problem

Hey all, I've had my computer running smooth for about a year now... I've recently come across this strange problem - it's rather hard to describe but I'll do my best.

Keep in mind my rig is cooled by an AC and temps never exceed 40c.

In games lately, (both 3d and 2d) my frames drop to 30fps or so for a certain period of time, I know when this is going on not only because my frames drop but i also hear a very high pitched sound in my headphones. I want to say the hard drive is trying to read something, but I don't see how that is possible as nothing is going on except me playing the game.

The high pitched sound seems to be the problem, should I attribute it to a disk drive trying to read or something?

I've heard cases of your boards on board sound fuzzing out,

does the drop in FPS happen at the same time as the sound?

Yes, they happen at exactly the same time, if I wait in a quiet area (in counter strike source for example) and watch my netgraph it shows a pretty steady 160 or so in dust2, simultaneously I hear a high pitched sound through my headphones and the FPS drops to 30.