A lot of graphic cards do this (yes even some ATI ones), and quite frankly since it's not an actual issue, RMA might not do anything. I heard some PSU do it as well (and in this case it's perhaps NOT normal).
The only solution I've found with my card is to limit the framerate in the games that allow it (or use v-sync which I don't really like in most games). But AFAIK, there's no actual solution to the problem besides pushing your card to its limits by throwing loads of AA, AF and all that.
But the sound does piss me off sometimes, a good example is when I launch Age of Conan : watching the cinematics with that sound if frankly not a pleasure. Because the game doesn't run well enough for v-sync and has no framerate limitation possible either :/
I heard someone who said he solved the issue by changing his motherboard
but actually he also changed his psu so how could you be sure... Maybe it's a combination of a specific card + a specific psu (and possibly + a specific mobo) that causes the noise. I'd be interested in knowing more but all I could find on google was some completely contradictory forum talk