Where s the noise coming from? Can you help please.?

Yesterday I was at my friend s house and she told me how she has this humming noise in one room of her house. It s right over the furnace. Her furnace is a hot water baseboard furnace. We went in the room you could hear the song slow quiet hum when the furnace was not on. It to me seemed like it was from the... show more Yesterday I was at my friend s house and she told me how she has this humming noise in one room of her house. It s right over the furnace. Her furnace is a hot water baseboard furnace. We went in the room you could hear the song slow quiet hum when the furnace was not on. It to me seemed like it was from the basement so we went into the basement. When we were in the basement we could not hear anything. Is brand new noise. She s been in the house for a couple years and has never heard it before. It s an annoyance noise. Does anybody know what it could be?
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