High-pitched noise coming from computer

Computer is ~3 years old. For the last 2 years, on hot days only (28C (82F) or more) it makes a high-pitched, quite loud, and very annoying noise. This noise comes in short bursts (up to 3 seconds long) and the occurrence frequency is unpredictable.

I've recorded a sample: http://www.box.net/shared/lqd3zc0ja4

The noise doesn't seem to be related to any particular activity, i.e. it occurs when the computer is idle, with and without high I/O activity, with and without high CPU usage.

I'm also sure it's not the PSU, as I've changed it and it's still screeching.

EDIT: it's definitely not a digital beep, it's an analog whine.

There is no crashing or freezing or anything abnormal when it screeches like this but it's quite annoying during summer. I've read (here, here) that it can be some electronic component in the motherboard gone bad. Is there any way to find which one is it? (carefully listening doesn't work, I've tried!) Is it even worth it? Is there any way to be relatively sure it's the motherboard and not the CPU or RAM or something else?

EDIT: I've now installed SpeedFan to see what's the temperature threshold where it starts making noise. Will update on the next hot day :)