I have a 21.5-inch, Late 2013 iMac running 10.9.5 that is around 2 years old. I've been having trouble with the fan intermittently running and not stopping. It's very loud and will run for long periods if left going. I've taken it to Apple and they say nothing is wrong with it. That it is 3rd party software causing it and that they want to re-install the system. I've looked in the Activity Monitor and not much is going on when its running. Sometimes it does it when it starts up so it's not like its running hot. I've tried unplugging it, running Applejack, CCleaner, and SMC reset. I see that there was a firmware update but when I try to run it, it says it is not supported on my system. I doesn't do it all the time so it's difficult to tell when it's fixed. Any other ideas?