My PC started making a whirring noise about 3 months ago which has been worrying me a bit... how can you tell which component is struggling? At first i figured it was the hard drive but it could equally be the PSU or CPU fan... how to tell??
either stick your head in it or use your pc microphone to listen to each fan through the speakers, kinda like a pc stethoscope :)
one of mine does the same as Genexi's... whirs for a few seconds on a cold boot... then goes quiet for the rest of the time...
I think this happens on most temp intelligent bioses esp. laptops, fan goes full pelt until the temp is registered by the bios and then is slowed down to save power.