It could be that your CPU fan is not connected to its designated 4-pin fan connector, but to some neighboring 3-pin connector which would make it run at 100% voltage all the time.
Check the BIOS as well, there is an option to turn cool and quiet on and off, and even switch between voltage / PWM fan speed regulation. One specific thing that you should also check is the "temperature target" (or something like that), if you set this too low, fan will work like crazy just to keep the CPU at this temp.