The hard-drive might definitely be audible when it's on, but it doesn't seem as though a "hum" would develop on a hard-drive that had originally been silent. A "hum" is not a word I'd use to describe pretty much any hard-drive I've heard. "Whir," certainly, "clicking" perhaps.
As for the CRTs, the same problem does eventually develop in most of them, but usually one's hard-drive or fan is noisier than the screen, and most monitors only hum when a video source is actively driving the beam. Otherwise the beam shuts off. So when the computer is off and the room is quiet enough that one could hear the hum, the hum shuts off too.
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As for the CRTs, the same problem does eventually develop in most of them, but usually one's hard-drive or fan is noisier than the screen, and most monitors only hum when a video source is actively driving the beam. Otherwise the beam shuts off. So when the computer is off and the room is quiet enough that one could hear the hum, the hum shuts off too.