Sometimes even a search on Google provides no answers due to the number of people asking the same question. The result is the collective knowledge migrates away from possible answers and is replaced by no knowledge, just myth. Now fans spinning fast on an iMac feels like a laptop from the 1990s, but here is low tech cause to a seemingly high tech problem. Someone took it apart, and put it back together screwing a case screw through the sensor cable. Its not like its mains voltage, so nothing as exciting as a spark fly out. All that happens is the sensor reads 1C or 150C depending on which wires got cut or shorted.
The SMC controller in charge of temperature regulation which has no reliance on the CPU or OS switches to safe mode. It doesn’t matter how many times you reset it, it will jump into that mode and spin one or more of the fans at a speed that will ensure nothing could ever bust into flames.
So if you find your fans spinning, and an SMC controller reset fails, check to see if any of the temperature readings are ridiculously low or high, and check to see if any of the wires are damaged. If you can get your hands on the Apple Diagnostic Software or Apple Hardware Test it may tell you which sensor is busted. In the case of many iMacs from 2007 onwards, its could be the LCD thermal sensor that passes directly over one of the holes for a long case screw.
Hopefully there have been enough words in this post to help anyone else at a loss to figure out why their family iMac sounds more like a 747.