I’ll defend this guy: there can easily be a harmonic at half the flyback frequency that is audible. It’s lower amplitude so less loud, but I could believe someone being able to hear that.
Yes, as I previously stated, if there is a problem with the flyback transformer circuit, it is possible that the frequency or volume of the noise it generates can become increased or different.
Though again, PC monitors never made an audible noise unless they were low quality and used the cheaper 15.7kHz transformer in their construction.
Other noises associated with CRTs are the degaussing noise, which only happens once usually after turning on the CRT or after pressing the degauss button, or the sound of old IDE hard disks spinning, which also make a constant high frequency noise.
Not sure you follow: even if the primary frequency is out of range, a harmonic (half the frequency, quarter the frequency, etc) can simultaneously exist with the primary.
I suppose its possible with a low quality CRT that has poor construction, leaving loose parts or thin plastic fins that can vibrate at harmonics of a lower frequency, but high quality CRTs don’t have this issue.
Extra or louder noise does not occur in normal operation of a correctly functioning flyback transformer circuit. Any frequency or harmonic (vibration) that occurs that is not the specified frequency of operation of the flyback transformer is caused by a problem in the circuit. A brand new CRT with known good parts will not have any kind of extra harmonic.
I had to replace a flyback transformer because it was too loud, and it was a pretty simple job. Hardest part was finding a new flyback transformer. After replacement, the noise volume was reduced to normal levels.