NUC 6i5 fan always on

I am running ROCK on NUC6i5 (dual 8GbRAM, M.2 SSD card and BIOS 60). Just recently I noticed that the cooling fan is always running, even with Roon at idle. It used to work fine for the first year or so.
I disassembled the NUC and cleaned the fan and heatsink (both were quite dusty). Also tinkered with a few cooling BIOS settings (performance, custom and quiet) but that did not change anything. While in BIOS the fan was spinning at 3200 rpms and the CPU temperature was 42C.
I have done an extensive online search but did not find any definitive/convincing solution.
I am considering a fan-less case (Akasa Newton) but they seem not longer/easily available (eBay, Amazon, etc).
Is there a particular reason that cause a cooling fan to spin regardless of CPU usage?
Any help will be greatly appreciated.