Did you upgrade the BIOS recently?
According to the BIOS release notes the default fan duty has been increased twice:
BIOS 036: "Changed default Fan duty to 30%."
BIOS 039: "Changed default fan duty to 40%."
I also find the fan noise of the NUC a little disappointing. It is not very loud, but a somewhat irritating high pitch sound.
It is possible to change the default back to 20% and you can also modify other fan parameters in the BIOS settings. But of course you have to be careful not to overheat the device.
Another post about the NUC fan that could be interesting for you:
NUC fan problem-high temperatures-simple mechanical fix-pictures included
Another option might be to replace the NUC case with one of the fan-less cases that are available now.