I've seen that a couple of times on my system if the input jack is not fully and/or correctly plugged in. What I get is pure 60 Hz hum. I find, when it occurs, that I have moved something and partially unseated one of the jacks. You might want to check to make sure this is not happening and/or your cords/jacks match and aren't giving you a phase to ground connection of some kind.
Re: my post immediately above this -- I guess I should not have used the term "phase to ground" as that sounds like I'm implying 120 VAC across the connection. What I really meant was some kind of current flow from power to ground and/or an inductance that causes/allows the ac sine wave pattern to be present at the audio input.
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