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 :)

If it's not something emitting from an actual built-in speaker on the motherboard or another part, I'd look next to moving parts. The only part that moves fast enough to sustain a noise of that high frequency would be your hard drive. Try running the following (Windows) command:

CHKDSK /R

This should scan for bad sectors on the hard drive, and attempt to recover information if any are found. If the scan does find bad sectors (a report is generated when the scan is completed) then this is probably your problem. Shut down the system until you've found a replacement drive.

Other than the hard drive, I don't know what might specifically cause that sort of noise. If it is indeed in one of the non-moving parts of your system, the only solution may be to try listening more carefully or swap out parts until the noise stops.

From the recording I can't tell whether it's more of a beep or more of a whine. Could you record a motherboard beep for comparison? Also a longer recording could help (is that really a second frequency at the end?).

If it's a beep, it could be your motherboard telling you something, or it could be a defect in the speaker. Try unplugging the speaker (in typical PCs there's a wire you can unplug somewhere; if you have the manual for your motherboard the right wire should be easy to find).

Otherwise it's probably a particularly loud electronics whine (I think it's too high-pitched to come from a fan, and too irregular to come from a disk). This can indeed happen to aging electronic components.

Did you open the case? This is pretty much necessary to get any clue as to the origin of the noise by ear. You might have some luck gently pressing your finger in some places and seeing if the noise changes. Mind your fingers though: don't touch any chip as they're usually burning hot, don't touch exposed conductors (you won't get electrocuted on a motherboard but your finger residues could cause a short), and don't apply so much mechanical pressure that you break something. Try gently pressing on edges of the motherboard, any extension board, RAM sticks...

It could be an alarm of some sort on your motherboard. Most motherboards have an audible alarms to indicate high temperature within the CPU. Check with the mobo manufacturer if it comes with a software utility.