SOLVED by 1440p HiDPI: MacBook Pro 16" is HOT & NOISY with an external monitor!

We’ve had a number of members complaining about their 16-inch MBP running hot with an external display. Here is a possible solution.

I am intensely curious as to what causes this problem. I have a 16" MBP with the Radeon 5500 and drive two 4K displays (scaled to 1440p) and keep the lid open for a third display, and I haven’t had unexpected fan noise of the sort described.

The inconsistency of this problem is concerning to me.