air ventilation in desktop is loud

You have Lion - is the computer new? If so call AppleCare.

Same problem here...

Brand new 1800 dollar computer,

Took it in ...Apple say this is normal.

I've had 5 macs and none of them have made this much niose...

i never had OS update with my 21 imac mid 2011, but since updated to mavericks i get loud fan noises from time to time for no reason whatsoever.