system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: That high pitched sound a tube television makes in which you can tell it's on even when there's nothing playing.

You all know that sound. You walk into a room where someone forgot to turn off the tv and you can tell right away it's on just by that really faint high pitched sound.
gpt
The tube television works by directing an electron beam to draw, pixels by pixels, scanning from left to right, row by row. Of course, after every row, the beam has to go all the way to the left again to start the next row. This means that the beam is constantly oscillating very quickly^1. The beam is directed by metal plates that quickly reverse charge. Electric current causes magnetic fields^2, and magnetic fields causes magnetic things to move around or even change shape! The high frequency vibration that results from this causes the air to vibrate too, producing the sound. 

Note^1 : Horizontal scanning is done at around 15734 Hz or 15625 Hz, which is near the upper extreme range of human hearing. Old people might not be able to hear this. Not to be confused with refresh rate, or vertical scanning, which happens at 60 Hz.

Note^2 : In particular, the thing called the flyback transformer, which converts our normal electricity to the high frequency sawtooth signal, is responsible for the noise since its magnetic coils concentrate the magnetic fields.