In the iMacs I have worked on there is a temp sensor cable that runs from the hard drive to the logicboard. You are right that the hard drive itself does not have a fan but if the iMacs use a specific kind of hard drive and there is a thin cable that runs from the hard drive to the logicboard that is called a hard drive temp sensor cable. If that cable is missing it will cause the fan to run at high speeds. I learned about this while working at an apple store. Look it up.