The extremely high-pitched noise you hear from old televisions is known as a "flyback transformer" or "line transformer" whine. It is caused by the electrical components inside the television vibrating at high frequencies.
Older televisions, particularly cathode ray tube (CRT) models, generate this noise due to the way they function. The flyback transformer is responsible for producing the high voltage necessary to power the electron beam in the CRT, which creates the image on the screen. As electricity flows through the transformer, it can cause the laminations and windings inside the transformer to vibrate, producing the audible sound.
The reason why some people can hear this noise while others cannot is related to the range of human hearing. As we age, our ability to hear high-frequency sounds diminishes. Younger individuals with better high-frequency hearing are more likely to perceive the noise emitted by the flyback transformer.
It's worth noting that modern televisions, such as LCD or LED models, do not produce this high-pitched noise since they use different technologies that do not require a flyback transformer.