Found this in Apple support forums. It's only temporary - fan will go back to high speed after sleep or shut down. But only takes a sec to paste into Terminal:
Install smcFanControl
Go to the terminal and use these commands to regain control of your noisy iMac:
/Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 12c0
This will set the CPU Fan to max at 1200 rpm: (Oh yes!)
To revert to standard settings use:
/Applications/smcFanControl.app/Contents/Resources/smc -k F2Mx -w 3cf0
This will set CPU Fan to max at the default 3900 rpms
You might have noticed that the last 4 digits are the fan speed (in hexidecimal)
Other fan speeds you can try:
1100rpm = 1130
1500rpm = 1770
3900rpm = 3cf0