When a power brick is humming is that a bad thing? What does it mean.
Does having it plugged into the wall directly or in a UPS or in a daisy chain of power strips make a difference
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If the power supply is not getting hot, there's probably no reason for concern:
As mentioned above by @Tetsujin and @Daniel B, the two most likely causes are loosening of the laminations of a transformer and the square-wave output of some UPS's. Since neither is significant, the noise can be ignored as long as the appliance works.
If you have a basis for comparison, you could make a judgement: if the power supply draws more current from the mains than before, or if there is AC leakage, as shown by an oscilloscope or audio amp, on the DC output, there might be some reason for concern.
Do not cover the noisy power supply in an attempt to make is quiet, though, as that might cause it to overheat.