I’d guess it’s on all MKIIs. I have two OTs. Both bought brand new. Both have the same exact sound. I’d also guess that some people cannot hear the frequency at all.
It is caused by the screen.
I keep brightness on low as a matter of preference, so the sound is very faint.
I’ve looked before and I can never find any info about OLEDs actually making noise, I knowthey can induce a lot of hum and whine in audio signals but nobody seems to say anything about them mechanically making any sounds, other than coil hum in OLED TV sets. But this thread and a few other things I’ve seen make me think that they can.