Stick your head in the PC and switch it on, try and work out where the noise is coming from then hit it with a large hammer (the area with the noise *not* your head!).
Silly suggestions aside, just break it down, unplug a few things and stall some fans with a paint brush (or fingers!) until you find the culprit.
Some stock CPU HSF are noisy, chipset fans are noisy, also some stock VGA fans are noisy etc etc.
Optical drives are the most noisy, them maybe a HDD during seek, everything else should be a low *hum*