Noisy CPU fan - reduce its RPM

If you do there will, of course, be less cooling effect, so your CPU may overheat. The problem may lie with poor thermal coupling between the CPU and its heatsink, so the fan is just doing its best to stop the CPU frying.

 

The question is whether it was always this loud, or has increased. It is not uncommon for laptop fan bearings to wear out or at least run out of lube, then there is not much you can do (unless you can relube it) besides replacement.

Slowing it down is not a good idea at all. Unless there is some other malfunction in the fan control circuit, it ramps up in speed based on CPU temperature so if it is spinning faster than normal it is because something is causing an elevated temperature, OR it could be as mentioned above, not spinning faster just beating around in the worn bearing trying to keep up and making noise doing that.

You should check CPU temp and if high, try to find the cause such as a viral or malware infection, or could just be a worn out fan can't keep up enough RPM assuming all air passages are free of dust.

Let's suppose none of the above applied, then yes in theory you can adjust the fans RPM down with a low ohm (tens of ohms at most) resistor in series. However the laptop should just keep trying to ramp fan speed higher from the result of higher CPU temp and that will only limit the maximum RPM the fan could have, which it should never be using unless the CPU is overheating, then it should be allowed to run as fast as it can trying to compensate, else you end up with a lot of heat buildup that can damage other things too.