Its an XFX HD 4870 XXX 1gb but the PSU is at the top of the case and the GPU is at the bottom. The noise definitely sounds like it's coming from the PSU at the top.
I found this video that shows the exact problem I'm having although it's with a different PSU.
http://www.youtube.com/watch?v=mQa79pWC0OQ
I noticed it when scrolling in IE but it wasn't too bad but in STALKER: Clear Sky it was much worse to the point where hovering over an option in the menu would change the pitch of the noise drastically.
Edit: Apparently it's called "coil whine".