Also, which is it? - a constant high-pitched whine (continuous) or a high-pitched beep (fairly short and repeats)?
Lots of possibilities : Modem; power supply, hard drives occasionally make beeping noises, your video card may have a fan, or could create something like a beep; the processor can emit strange noises; your system should have a small fan on the processor that can get noisy also.
If you drop open the side door, you might be able to localize the noise. This is perfectly safe, as long as you don't start poking around inside,