Corsair HX750 Hissing noise

Its only been an hour and a half, that's a pretty good response time compared to most forums  

Sounds like coil whine to me, and it seems to only happen when under a heavy load. Download a program called Furmark, and run that. If the coil whine happens, then you know its either the PSU or GPU making the noise.
Then download a program called Prime95 and run its torture test, if the whine happens then you can be fairly sure its the PSU. If it doesn't, then its back down between the PSU and GPU. Will have to figure out some other way to establish which one it is, maybe move the card to another system and try Furmark again.

Whichever component it is, see if you can get an RMA on it.

I have this as well, but it's my graphics card that does it, not the psu. It's perfectly safe and fine, but can be extremely annoying.

What I do is limit the framerate to 60 using msi afterburner. Some games without vsync will be rendered up in the thousands frames per second (usually 2500+ and usually in menus and intro videos only). Not only does this make the graphics card much hotter, it also produce a coil whine after a while or instantly.

I really don't think it's your PSU.

Its noticable in menu of games and when not using vsync! i m seeing that it is pretty common with corsair psu's! If its really safe & fine then I wont need to RMA it!

But can the coil whine increase over years?

I have my i5 3570k OC ed to 4.2ghz with auto voltage too! maybe it can be the cause? or?

Thanks guys