I was just asked by a long time friend what software I recommended him to install on his fairly new iMac to control the fan speed. As many iMac owners know after swapping out your hard drive for a fancy solid state drive or just replacing your bad hard drive with one off the shelf, some new problems occur. Apple has installed hard drives with proprietary temperature sensors in them and when you replace that drive with one not compatible your fan picks up to take off speeds. Super loud and annoy, just for replacing your hard drive or upgrading to a SSD. Apple's SMC, system management controller, monitors sensors in your iMac and adjusts certain devices based on the input it gets. Thus a hard drive not reporting, which reads as a very hot hard drive to the SMC, means crank the fans up! The simplest solution without opening the machine back up is to install a small application that allows you to control the fan speed. Best yet the software I recommend is free and can read SMART data to help monitor hard drive temperatures without the need of a proprietary sensor. There are a couple other suitable options out there but from my research and testing this was my favorite and the one I suggest to my friends. Give it a try, if you like it consider donating to the developer.