CPU fan run at 3200 or higher for no reason

I have a 2011 iMac and for a while the CPU fan goes at high speed for no reason. I installed Macs Fan Controll but it does not allow me to reduce to the desired minimum speed ... only when i increase the temperature levels the speed goes done a little bit ,,, i.e. from 4000 rpm to 3200 rpm ,,, of course i did the rest, unplugging, pressing the power button etc.

any idea what i else i could do ,,, i looked on the internet, some solutions worked for others ,,, but they dont work for me ,,, non of the fan speed control programs work ,,, now i have installed iStats menus 4 ... the trial version ,,, it worked for 30 min ... i was so happy i was ready to buy the program ,,, and thought i better wait some days ,,, the version is free for 14 days ,,, but after 30 min back to 4000 rpm ...

what should i do? I just moved to Warsaw and no apple store around ...

thanks