Having done that if that does not fix the issue I have heard there is some sort of setting to get rid of that noise (I heard of this type of problem before) its either turn the fan up higher so it keeps things running cool.
Or go into your power settings in 7 and turn off your sleep settings and turn your system to "always on" I know this is a less then ideal solution and is definitely not green lol but it may work untill things get worked out.
I think it is an issue with Nvidia's driver and they will get it fixed in no time
Edit: Ooops yeah if its your mobo check to make sure your fan is still spinning on your CPU and check your temps ASAP... could be bad things happening.
Sounds like a power issue for your graphics card. Check that the additional power connector is secure. And if this only happens after an hour or so I would suspect the PSU is not capable to delivering enouhg long term power
Well the PSU died, and i thought the motherboard did aswell.
Couple of days ago a brand new PSU turned up and the motherboard came back from being tested so its all ready to go again.
Rebuilt it lastnight paying great attention to detail to make sure everything was fitted correctly.
Powered it up, worked fine, 30-45mins later the system went to Sleep... and the graphics card started screaming like a banshee yet again.
I'm away from home right now, but when i get in tonight ill put the system to sleep and see exactly whats happening with the fans incase its some kind of fan-alarm deal.