I had a problem and found the solution, so I’m posting incase anyone Googles and come across this. 
Ever since I formatted my computer, I’d hear a constant high-pitched sound coming out of my headphones (very faint) and it’d change in pitch every time I moved my mouse. Turns out muting my line-in in my sound control panel fixed it. Some type of interference I guess.
Hope this helps anyone having the same problem.
It might be handy if you post what kind of motherboard you have?
http://www.viper007bond.com/about/my-computer/c2d-e6750/
It sounds like the classic CPU RF interference problem. I have it with my laptop, and there’s a number of factors involved (poor quality switching power supply, my CPU leaking Radio Frequency interference into surrounding circuitry and a poorly shielded audio interface).
I found if I isolated my power supply from the ground – effectively lifting the earth, a pretty dodgy thing to do safety-wise – the noise all but disappeared. However, it can happen with many, MANY onboard chipsets on desktop motherboards, and there’s little you can do sometimes. However, consider changing the power supply if you have a budget unit inside the machine.
Also, make sure your PC, audio equipment is all on a common ground – this can help. Also, try running your PC through a UPS (which will filter the incoming mains AC and generate its own, far purer, AC current).
It’s a bugger trying to get these sounds to go away though! I’ve known people to drive copper stakes into their garden (literally, earthing directly) and setting up an entirely isolated ringmain just for their audio equipment…
I’m on a UPS, it’s an old Audigy 2 ZS audio card (not onboard), and an Antec NeoPower 650W PSU from a few years back.
Oddly it didn’t do any of this pre-format, although I was using the basic Creative drivers from Windows Update at the time of this post.
Curious. Have you tried the kX Drivers? I think they’re compatible with the outboard Audigy 2… (I have a PCI Audigy 2 Value, it’s never let me down all these years and its ASIO performance is outstanding with the kX drivers! I’m using the ProFX drivers for better I/O but I used stock kX for years with no complaints).
Muting the Line In, if you’ve got nothing plugged in, sounds like the RF intereference from your CPU’s still leaking into the circuit somewhere – or you have an unbalanced circuit somewhere, a loaded capacitor or something. (has your signal flow changed outside of the ZS at all? Any impedance imbalances?)
Have you tried just plugging in any old device into the Line In, even if it’s turned off, and seeing if the noise persists?
The noise has stopped now that I muted my unused line-in, so I’m not terribly worried about tracking down why it was doing it. I’m just happy it’s gone.
But for how long? …