Comment 1 for bug 1442416

Gordon, are you still using this computer and still having this issue? Did you find the cause and/or a solution?

There are other reports of macfanctld not working as the user wishes on early-2006 to mid-2011 iMac models. I believe most of them stem from the fact that these models have three fans (HDD, ODD, CPU) and macfanctld can only drive two fans. If the CPU fan is not among the two identified fans, computational load can drive the CPU into overload and make the SMC drive the fans at maximum speed.

Or did you change the hard disk drive and forgot to put the HDD temperature sensor in place?