I seem to have the opposite problem that everyone else has with PSU squealing.
My PSU, an Antec Neo ECO 520C, squeals very faintly, but very high pitched, when there is absolutely zero load on it. As long as there is more than ~5% load on any one core (i7 860) this noise goes away completely.
Turning off C1E makes this noise go away also while the system is idle, except when I move the mouse cursor under otherwise zero load, then you can barely hear the faint but very high pitched squeal again.
Clean all the dust out of the unit and if your handy you can replace the fan. There is also a way that you can oil the fan shaft but this will only buy you a little time eventually it will need to be replaced.
We've had a few similar threads in the past involving psu noise and mouse movement. There are several possibilites:
1. Deselect Smooth Mouse Scrolling in Internet Options.
2. Disable, turn off, or mute any unused recording inputs
Shovenose ready mentioned a deteriorating capacitor or coil in the psu. Deteriorating capacitors are the most common cause of electrical noise. The capacitors are used in noise suppression circuits. However, the actual bad capacitor may not necessarily be inside the psu. It may be on the motherboard, video card, or other component. Look for a capacitor that is bulging or oddly shaped. It also may have an ugly bownish sticky fluid that leaked out. They are easy to spot.
The noise is definitely coming from the PSU itself. I can't see any leaks anywhere either. It's also ONLY when the PC is under very low loads, the noise completely goes away when the CPU or video card are doing anything. It's not just the fan noises masking it.