I suppose it's some sort of resonance phenomenon.
There's specific test images you can find online, designed to maximally stress voltage rails in LCDs. Lower end monitors can actually get enough voltage ripple that the image quality visibly degrades.
Naturally, its the speadsheet that I use most often in my duties.
You can dial the screen's volume by making the libreoffice window partially transparent (I have that bound to scrolling on the title bar). This is on an Acer 1920x1080 (~23"?) screen from around 2009.
I have a feeling that grey #A5A5A5 is row-hammering something
I can occasionally hear such noises from monitors, and have always thought it was some kind of interaction with the phase of the AC power and some kind of internal physics of the monitor. Generally, turning it off and back on again fixes it for me.
But as this article describes, you never know!
I don't honestly think turning it off and back on would do anything, either. I also don't have one of those chairs, nor do I want to buy one and use it to test the theory. But, turning it off and back on again is a simple and easy thing to do that should be reasonably safe for the equipment. There is, after all, a reason why power cycling equipment is often a first step to diagnosing and/or fixing weird problems. :)
When we were teenagers, my friend used to call it "waving a dead chicken". He coined this term to describe the way he would resurrect dead inkjet printers that even I gave up on - by disassembling and reassembling them until they started to work again, while being perfectly open that he has no idea how it could fix the problem, just that in practice it often did.
While this was just a funny term and pretty absurd approach for fixing things (even though it worked!), I took away from it an important realization: the scope of possible causes of a weird, randomly-occuring problem is much larger than I'd normally assume. Over the years, I learned to identify some "outside context" things for computers - ESD, thermals, UV exposure, RF interference, voltage spikes in power lines, devices being almost but not quite connected. Because of that, when in a bind, "waving a dead chicken" may just be called for - in forms of e.g. percussive maintenance (hitting the thing with a wrench), moving things around, switching cables, disassembling, etc.
I'm not entirely certain, but as I recall it switching it off and back on again is not effective against this issue. I've tried, no doubt, but the only thing noted which seems to have an effect (while the screen is active) is removing the text from display.