Don't ever adjust the speed of the fan down as you risk having the laptop overheat once the fan is running normally. You can try booting into Safe Mode to see if that makes a difference in case you have some third party software installed which is interfering with the normal operation of macOS.
Try an SMC Reset.
Try running the Apple Diagnostics since most likely the laptop has a hardware issue of some sort. Most likely it will be a bad sensor on the Logic Board since only one fan is affected, or it may be the circuitry which controls the fan. You will most likely need to have Apple or an Apple Authorized Service Provider examine the laptop to provide you with a repair estimate.
FYI, this laptop will be going "Vintage" at the end of the month which means the laptop only has at most two more years of hardware support available as long as the necessary parts are still in stock. Plus the next major release of macOS (Ventura) later this year will not be supporting the 2016 laptop so this laptop will stop getting security updates for macOS 12.x Monterey in 2024. Keep this in mind when considering repair costs.
Make sure to have a good backup in case the Logic Board fails completely since it may not always be possible to recover data from the SSD which is soldered to the Logic Board.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.