I've experienced something like this before. Mine were high pitch tones I heard through my headphones while I would open a window or use the scroll button. Almost like I could hear the frequencies and call signs going through the board (bare with me here, this isn't some Art Bell ramble). It was lack of shielding in the common rat-tales. My Belkin KVM switch was resting on my mighty8 surge protector power cord. I guess I had some kind of minor interference coming through my VGA connector into my video card and on to my board and then into my soundcard and out to my headphones (does that rhyme)? In the end I ruled it down to a lack of shielding. But this wasn't my only case of weird noises that I've solved. I needed sound on my libranet box so I bought a cheap-ass $20 soundcard from Comp-USA; this was a piece of crap a real work of craftsmanship out of China. What they didn't tell you on the package is that you'd be spending 30+ minutes finding the F-n drivers on the Internet just to get it to work. (I'm probably the only customer they had that didn't return this model card back.) Anyway, It turns out the bracket is warped and it's slightly touching the Mobo and giving off a high-pitched hum not a whistle.

Anyway hope it sheds some light.