hhuh may have something - try unplugging your speakers and disconnecting the system speaker from the motherboard header to see if the sound persists...
I literally just stuck my head inside my case and it is one of three items:
1. Corsair HX650 PSU
2. ASUS P7P55D-E Pro Motherboard
3. Ati Radeon X300 GPU
Unplugging the speakers did not change a thing. My gut is telling me that I'm hearing it coming from inside my power supply.