Thanks for the replies. I may try playing with this on another rig to test out those PSU and / or motherboard theories. I'm a bit wary of plugging this brand new but not exactly shiny card into a CoolerMaster ElitePower 400W though (the only other PSU I can play with at the moment).
Oddly enough the card doesn't whine at all with OCCT GPU test going, despite the card reaching higher temperatures than with folding. For games I haven't tried as much because my closed-back headphones almost completely drown out the whine (even with no sound playing through them). I'll see if maxing out the card's OC has any effect on the card.
Anyway I won't return the card over this because it's still an improvement over my Radeon 4870 which sent its electrical noise through the headphones
At least this card doesn't do that... Now if the constant F@H while OC'd to 1GHz core happens to kill the card over the next week or so that's a different story
