You can adjust fan behavior using additional software as per below:
iMac users who tweak their systems using 3rd party hard disks or solid state disks, are often surprised by an unpleasant aftermath: an annoyingly noisy hard disk fan. Fan management in Apple systems relies on temperatures reported by the SMC system. The SMC collects data from external sensors or by reading the internal hard disk temperature in a proprietary manner. Sometimes reusing the external sensor is not possible or, as in more recent generations of iMacs, replacing the original hard disk breaks the system’s temperature monitoring. In both cases the SMC system tends to go into frenzy and drive you mad with a loud fan.
SSD Fan Control puts you in control. It enables you to overrule the automatic fan control and manually configure the fan speed. The core of the system is a daemon that runs in the background and takes care of the fan management. Whenever the SMC system resets to its default, SSD Fan Control takes over again to make sure that your system remains quiet. Aside of this daemon, SSD Fan Control features a user-friendly application for monitoring and control.
1. Download the package http://exirion.net/ssdfanctrl/
2. Run the installer (reboots your iMac after finishing)
3. Run the SSD Fan Control application
4. Find the noisy fan and control it in ‘Manual’ or ‘SMART’ mode
5. Enjoy the sound of silence