Just in case you don't know, if you record couple of seconds of that hum in front of the work being recorded then select that piece in front of the work and in effects click sound removal/get sample, then select the whole piece and go to effects again and hit noise removal/remove noise, it will take the hum out right through the record.
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keybards with transformers on them sometime emit a bit of a hum too.
Or if you aren't using the line in, or the mike, mute whichever one you are not using helps somethimes too.
Graham