If you're not already doing so, use
iStat Pro (free) or
iStat Menus ($16) to get accurate readings of your temps, fan speeds, etc., rather than relying on your sense of touch or sound. A forum member has posted a copy of iStat Pro that has been "tweaked" to enhance compatibility with Mountain Lion. You can download it
here.
Your fans are always on when your Mac is on. iMacs have 3 fans with minimum speeds in the 800-1200 range. They will spin faster as needed to keep temps at a safe level.
If your fans are spinning up without increased heat, try
resetting the SMC.
(PRAM/NVRAM has nothing to do with these issues, so resetting it will not help.)
Learn about the fans in your Mac