I've had this problem with my 27" 2010 i7 iMac for over a month now where even light usagee will cause the CPU fan to spin at over 4200 RPM. I don't think the temperatures (in C) are particularly high or should be a concern, but the loud fan noise is constant and starts up soon after turning on the computer. I've tried doing a SMC reset, physically cleaning the vents underneath the iMac and I'm in the process of reinstalling a clean copy Mavericks right now (for a different reason). Could this be a hardware issue? Below is a picture after a few minutes of light use (copying files over and using Safari).