I am getting this sound too, but not constantly. It comes in waves, building up slowly in intensity, then fading away again. It's not really loud, but it penetrates your head.

I can't really tell if it's coming from the PSU or gfx card. When I put my head near the PSU to listen, the sound is drowned out by the whirring of the fans.

My current psu is a 500w Enermax Liberty, with dual VGA 6-pin PCI-E connectors, so it should be powerful enough for a single GTX. I was going to get a new PSU anyway, probably ~800w, so I'll find out if it's the card making the noise when the new PSU is installed.