A bad capacitor can emit a screeching sound, but not for long. You probably don't have any capacitors big enough to do that.
CPU fan, system speaker, power supply fan, other fans if present, should be checked.
Also, it could be the hard drive creating the screech; bearings, or head to platter contact. If it is the power supply fan, replace the power supply.
The CPU fan is the only item I can imagine screeching on the MoBo, unless you have an AGP card with its own cooling fans.
I would be interested to hear what you discover. I will bookmark this page.
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