The whine started getting worse after the 2013 MP was out of warranty, so not much I could do about the noise. I don't know how common this issue was on those machines. Thinking back, my late 2013 MBP also had coil whine issues. That was annoying too, but got fixed when I had to get the logic board replaced under warranty due to failing video card while under warranty.
I can hear the squeal faintly when I hold my ear up to the front of my M1 iMac on minimum brightness, but I can't hear a thing from my sitting position 2 feet away from the screen. I must have gotten one of the good ones? I hope it doesn't get louder as the years go by.