Computer won't turn on, emits a high pitched noise

Hi, my computer won't turn on, I thought it might be the power button, so tried screwdriver on the 2 pins, but that never worked, then thought it might be the psu, so shorted it using a piece of wire on the 24 pin connector, which fired the computer up and started the fan etc. So don't really know what else it could be now, any ideas? It's an Acer Aspire M3802 Desktop
Cheers
 
The computer won't power on to check any beeps for the POST test, the high pitched noise is really faint, seems to be coming from the motherboard near the 24pin connector?
 
8 year old machine? possibly voltage regulation chips on motherboard or capacitors. Examine under strong light for brown or black smuts or bulging capacitor tops.
 
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