OK at this point we know the issue is not a thermal one but either a voltage issue or a bad fan.
Lets try running diagnostics to see if you get an error in the power sensors. Restart your system and press the '''D''' key. Let us know if you get an error.
Reference: [https://support.apple.com/en-us/HT201255|Mac startup key combinations]
If you didn’t get an error then its time to replace the fan unit [product|IF188-054] and here’s the guide [guide|40700]. What likely has happened it the tach feedback long within the fan has failed.