Reset the NVRAM/PRAM which is more likely affect the fans than reinstalled macOS.
Done that and cleaned a little dust out of the fans. Anything else?
Run the Apple Hardware Test to see if it comes up with anything.
How fast do they come on when you power on the Mac? Has it always been that way? When did it change?
I had the same issue when I replaced the internal HDD in an iMac with an SSD - the Mac got bad (or no) temperature data. Macs Fan Control solved it.