Weird High Pitched noise.

Might be a fan that's scraping against something, as a lot of fans blow harder when the system's hot and blow less (or not at all) when the system's cooler. That would explain the sound occasionally going away. Exact same thing happened here, it was the fan of the video card making this really annoying noise now and then. I would suggest opening your computer's casing and starting it, listen really good to hear where exactly the sound is coming from. If it turns out to be a fan, you could maybe carefully suck the dust out with a vacuum cleaner, or oil the thing. This might not work though, and if that's the case, you'll have to buy a new fan.
 
I think it has to do with my firepod hooked up to my comp, but I don't know EXACTLY where its coming from it's like a 15-20k constant. It usually happens when I start up my gear and go into cubase and won't stop till waaaaay later/.
 
My dell laptop makes some weird beep beep beeeeeep beeeeeeeep beeeeeeeeeeeeeeeeeep beeep noise too. I always wondered what it was. It's not very loud...you can barely hear it when the computer's fan is on. And when the computer is processing something it goes away while it's processing then comes right back. I've tried kicking it but it didn't help. Weird...
 
My PC makes an High pitched humming that builds up (sounds like its gonna explode).
Only when i start loading a lot of plug-ins and the CPU starts straining though.
Maybe check your CPU usage if its not high?