I was able to test my new HX850 today as well, as my new memory arrived today. But things just got weirder, the sound is still there! It doesn't seem to be coming from the PSU anymore though. This time it's coming from the CPU socket area, I'm quite sure. I have to say at a certain point you stop trusting your ears, like colorblindness.
HDtune benchmark: definately audible sound
windows/bios idle: none
Haven't tested anything else, but I assume the sound will also pop up from other types of load being thrown at the CPU/PSU/board.
Right now, it's still just an annoyance factor (though I didn't pay 200 euros to have some funky business going on with my board...). I will try to use the system as much as possible. Should I try to fiddle around a bit with C1E/EIST, this is not quite clear to me. Or perhaps contact Gigabyte and have them to look at the board and fix it the proper way, for once and for all.
I mean, having RMA'd two kits of memory and a PSU, lets throw in the board itself as well!
And a happy note: not dissapointed with the new PSU at all!