According to Apple Knowledgebase article TS3334, after applying the iMac SMC Firmware Update 1.0, some late 2009 model 27” iMacs may experience a problem where their fans continually run at full speed.  Their fix is the reapply the iMac SMC Firmware Update 1.0 again, and if that doesn’t correct the problem to have the iMac serviced by Apple. http://support.apple.com/kb/TS3334