Might be a piezo thing. I have the cooling fan removed (still plugged in though) and listening through a straw to try to pick out the exact location points to the CPU, which I find hard to believe. It's a pretty loud noise too, louder than a fan - a veritable squeal. I've popped out all the cards (8800gt, modem(!), tv.) And popped the memory out too. Still the same noise. Put in a prior original cpu (1.8 ghz) and it makes the same noise and no post. Power up without the CPU and it makes no noise at all. Next step is to substitute a known good power supply... maybe a rail's gone down. All the caps look good.
Never heard this noise (loudness, frequency and location) out of a computer before... and I'm an old fart.
Mobo's an HP/Asus P5BW-LA(Intel Socket T LGA775, P965 Express, Micro ATX, 4GB DDR2, 1066MHz FSB)
Thanks the the tip Mondo man.