I recently discovered this page (epilepsy warning; flashing lights), which displays horizontal black and white lines and animates their thickness. Amazingly, it makes my laptop LCD screen emit an audible continuous squeak that changes in pitch. It has a few thousand upvotes on Reddit, implying this is a common observation.
For experiments, I created a large image with 1-pixel horizontal lines, and one with vertical lines. Try zooming in. For me, it's loudest at 200%. Interestingly, the vertical one is silent at any zoom level.
drivers99 on Hacker News speculates:
Without knowing anything else, I guess that there may be a capacitor somewhere that is charging and discharging along with the brightness of the screen as it is refreshed from top to bottom, which is causing it to flex in a way that produces an audible noise.
Is that a workable theory?
Why does it only happen for horizontal lines?