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There is one particular type of hum that has nothing to do with RF or ground loops.
It is generated by the computer itself.
When interfacing with outboard equipment to and from the computer’s sound card, the external equipment may have too much input sensitivity, thus it is picking up internal noise in the sound card itself. This hum is like a spike type hum, in fact when viewing it on an o-scope, the waveform is NOT a sine wave hum, but more of a saw-tooth type waveform, and sounds almost as if the hum has a high end spike type sound with some noise floor added.
Typical fix to this is to use 1 to 1 isolation transformer or a ground loop eliminator box like those found at Radio Shack. Though it is a ground loop eliminator, it is actually functioning as an isolator for the “hot” side of the audio connections, and of course as well as the ground side of the audio connections. But it does eliminate that high pitch type hum.