1. Update your NUC to the latest bios version 0050: https://downloadcenter.intel.com/download/28185/BIOS-Update-HNKBLi70-86A-?product=126143 Download BIOS Update [HNKBLi70.86A]
2. My NUC8i7HVK with updated bios and cooling mode set to Balanced (Fan OFF capability disabled), run noiseless. While I'm browsing in Internet or seeing videos (also 4K), both fans run at very low speed: about 650 RPM and the CPU temperature is less than 50 deg C.