I was doing some testing today with my XFX 7900GTX and noticed something during a temp test with ATiTool..
On 2D mode all is well but as soon as I hit the artifact scanner and even more clear when you just hit the Show 3D view there's this high pitched noise coming out of the back of the card where the volt regulator is located.

I had my case open and have a very silent system and it spotted my attention immediately... Although for some you might need to listen up close..
I know nothing about electronics but it sounds like some sort of interference of some sort... maybe a capacitator or something..

Can you verify if the same noise comes from your card...?