I’m reporting a solved problem, just in case anyone else gets this in future.
I have a iMac 27" mid 2011 (iMac12,2)
This particular model came with the option of an SSD and HDD, the lower priced versions had only the HDD but there was space for two. I bought the smallest HDD and immediately opened it up to add an SSD and larger HDD.
The side effect is that the power Y cable required doesn’t carry the temperature causing the default state to be all fans on 100%.
The solution was to download an app to fix this and manually set the fan speeds. I have been using smcFanControl for many years successfully and although I moved to Sierra some time ago, it only just stopped responding. Maybe there was an update that broke things.
I found MacsFanControl which has returned the jet pack to a normal level. The only caveat is that it doesn’t have the nice presets of smcfancontrol, you have to adjust the values individually.
Hope this helps