Well if nothing else is connected and you do not even have a cable box connected i would say it could be the TV itself. Could you try plugging it into a neighbors house outlet and see if the buzzing is still there? If it is not there, then it is not the TV and you need to get an electrician to check the house wiring or maybe simply run a dedicated line from the breaker panel as the cheapest solution that doesn't just mask the issue. When a power supply buzzes that usually means it is under stress.
Try plugging it in at a neighbor's house to see instead of bringing it back to the store to confirm it is your house wiring that is the problem.
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