You will find HDD Fan Control in your downloads folder.
It's both the demo and the full version (with license):
You can try it out for 1 hour after installation
then you must purchase a license to unlock the full version.
Instructions and Help for HDD Fan Control can be found in Support.
Reduce Fan Noise and Protect your Drive.
A post on Cult Of Mac reporting on the hard drive replacement fan issue in the latest iMacs shouts out HDD Fan Control as a solution, read the article here 2011 iMacs Make It Impossible For You To Upgrade Your Own Main Hard Drive