Currently Being ModeratedFeb 18, 2012 10:42 AM (in response to marsier)
Which model iMac?
What OS X is it on?
What App's and backgroung process are running?
Currently Being ModeratedFeb 18, 2012 10:47 AM (in response to marsier)
The fans running are a symptom, not a problem. They come on in response to a perceived issue.
You could use iStat Pro to see how many fans you have and which of them is running fast ... and then check the Activity Monitor (in your Utilities Folder in Applications) to see what out-of-control process is generating the heat that your fans are responding to.
You don't indicate which iMac you have but my 2.8 GHz model has 3 fans that run at 700, 1200 & 1200 RPM and never rise above that unless called upon in response to a situation.
A place to start, anyway.