Page "Cathode ray tube" Paragraph 23

CRTs used for television operate with horizontal scanning frequencies of 15, 734 Hz ( for NTSC systems ) or 15, 625 Hz ( for PAL systems ).

These frequencies are at the upper range of human hearing and are inaudible to many people ; however, some people ( especially children ) will perceive a high-pitched tone near an operating television CRT.

The sound is due to magnetostriction in the magnetic core and periodic movement of windings of the flyback transformer.

Compare to the low-frequency noise ( 50 Hz or 60 Hz ) of mains hum.