Chosen Solution
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)
Sounds like your display needs a dusting out! These are great displays! But over many years of use the dust buildup inside can interfere with its cooling. You may need to replace its fan if the bearings are gone Apple P/N 922-9358. To open the display its just like the 2011 27” iMac, follow this guide iMac Intel 27" EMC 2429 Display Replacement Use a small soft paint brush to help scrub the surfaces to loosen the dust and then use can’ed air to blow it away.
That’s by design. MacOS expects the (i)GPU to be stressed with a external display attached. Like you would use Final Cut Pro, or some other rendering software. Also the thunderbolt compartments (internally) tend to get hotter while in use. We used to be able to control fan speeds, but with the new T2 Security chip that’s made impossible.
