High pitched whine from newly built PC

I've just built a new PC:

• Asus P6T Deluxe v2 motherboard
• i7 920
• Sapphire HD 4670 Passive heatsink
• Corsair 620hx portal supply
• Antec P183 case

Sometimes when I turn it on, there is a high pitched "electrical" whine that comes from the PC. It doesn't happen all the time. Sometimes the PC is quiet after I've turned it on. During boot, the pitch gets higher or lower, depending upon what is currently on the screen, e.g. BIOS post screen might be higher than a black screen.

Its not the fans – I've stopped each one and the whine continues.

I don't think its the power supply – it sounds louder when I put my head near the main chamber of the case.

So I think its either the video card or the mother board, but I'm not 100%

Has anyone any idea what might be causing this, and how I can determine which component is causing it? I don't have any spare components to swap in and out to check this, and I don't want to guess at sending parts back to the supply.

Thanks in advance for any suggestions.

UPDATE: It feels like to me that this only happens when I've attempted to set up RAID1 on two of my disks. I've not been successful setting up RAID yet (that's another story), but I've disabled RAID at the moment since I'm fed up trying to get it going. The PC is not whining at the moment. My gut reaction is that this has only happened when the machine had RAID set up. I'm now going to run for a while with my non-RAID set up and see if I get the noise or not.

Thanks for all the suggestions so far.