my fx5500 used to do that. It wasnt getting enough air. Anyways, im getting in a new 6800xt, and i hope it doesnt do that. ps, if I put my hand over the blowhole fan in the top of my pc, the noise stopped. So, that means that it was probably the card not getting enough airflow, because without the blowhole, the air was not being taken away from the video card, and the noise stopped. Try geting a pci fan, like this one:
It probably wont help a whole lot with cooling, because it just blows warm air back to the card. But it should stop the noise.
And dont get an exhaust fan, because that will take air away from the card.