Good ideas so far. I hope the friend’s studio uses a dedicated circuit. Otherwise things like a furnace, fridge, air conditioner, ceiling fan, even a florescent light can do this. We had one case where an AM radio station down the street caused it. Solution on that one: move.

Steve suggested ground loop. Get a $10 outlet tester and check every outlet that audio equipment could be plugged into, since cables connect the various devices. I always try to plug all connected audio into the same outlet.


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