Not sure about the cause here but Dh replaced our electric hot water tank the other day which is also connected to a recirc. pump. Since our "mechanicals" are all located behind our bedroom wall we could hear this humming noise in the night(not constant, periodic). I would get up and go out to the kitchen thinking it was the refrigerator but no sound; then went into the mechanical room and heard this humming. First, we thought it was the new tank but then Dh put his hand on the copper pipe going to the recirc. pump and it was vibrating. Did he miss something by hooking up the new tank? The water seems to be hot everywhere but for sure, this humming and vibrating is new! What do you think is the problem?

Thanks!