Fan always running with loud noise.
I recently bought a second hand 27 Cinema Display to connect to my MBP(Mojave). After I connect the usb and TB2 to my MBP. The monitor works fine, but the fan running at full speed for about 3 sec and then start to slow down until it reach to a steady point but it still noticeably loud (sound like about 4000RPM). I try to adjust the brightness to minimum and the fan is still at the same speed. I switch to another MB Air(High Sierra) and it works the same. I wonder what might cause this issue and how to fix it.
(P.S: up to day firmware)