In the comments there are two pertinent comments that stand out to me, one suggests they stopped the fan and the noise continued assuming transistor whine as the source of noise and another saying there is definitely a second fan in the monitor that turns on after a while, which tallies up with the suggestions that one fan is always on and the other noisier one is the one for the gsync unit and that it operates based on temperature.
I am noodling here, I don't know but there seems to be a few questions that remain open.
Cheers for the links btw, I had seen them before (noted the fan model details in this thread a bunch of pages back) but very handy to have them in this discussion!