My Brand New CTC-7!

[Electronic M]

If the hum is only when receiving a signal, then it is an effect of the signal and not the power supply filtering. It will probably be worst with high contrast/fine text on the screen. There are a number of causes for it. A predominant one is video over modulation....The audio is FM the video is AM...when AM over mod occurs it also inverts the FM carrier and this shifts it's phase by 180 degrees (a form of phase modulation). FM and phase modulation are so closely related that a detector for one will receive the other (and vice versa) with an integral or derivative operation preformed on the recovered modulating signal.
This PM audio carrier distortion becomes an annoying buzz when demodulated. The only solution is to turn video modulation down (if you have a modulator that supports this such as a blonder tongue AM series), or get another modulator.

There is also the potential of audio misalignment. which can accentuate this effect on some sets.