It might happen with non-crt screens too. I remember a flat screen (LCD?) that made a different noise depending on the color it displayed. White and light colors made a lot more noise and if you had good ears you could tell the difference without looking. Not sure how they work though to explain this.
The inductors used in the power regulation circuitry physically vibrate due to the electromotive force the part relies on to function. Changes in the load on the power supply changes the characteristics of the vibration, allowing audible detection of the variation.
The physical vibration slightly alters the electrical characteristics too, which is why inductors are glued down or “potted” in some equipment to try and negate this effect.
Edit: The inductors on your graphics card can whine too. This generates noise on the ground line of the whole PC which is then amplified by cheap sound devices, which is why you can literally hear the mouse moving on the screen on some PCs.
It might happen with non-crt screens too. I remember a flat screen (LCD?) that made a different noise depending on the color it displayed. White and light colors made a lot more noise and if you had good ears you could tell the difference without looking. Not sure how they work though to explain this.
“Coil whine”.
The inductors used in the power regulation circuitry physically vibrate due to the electromotive force the part relies on to function. Changes in the load on the power supply changes the characteristics of the vibration, allowing audible detection of the variation.
The physical vibration slightly alters the electrical characteristics too, which is why inductors are glued down or “potted” in some equipment to try and negate this effect.
Edit: The inductors on your graphics card can whine too. This generates noise on the ground line of the whole PC which is then amplified by cheap sound devices, which is why you can literally hear the mouse moving on the screen on some PCs.