Had a near identical issue 3 years back in my apartment! After tracking the source (ear on walls, floor and just figuring out where it was) i ended up discovering it was an alarm in the mechanical room below my apartment. There was a crack in the heat exchanger and it was a CO alarm, the disturbing part (lost sleep aside) was that my complaint about the sound is what alerted the building to the issue, and worse yet, their solution was to consider it a false alarm as they couldn’t see anything wrong and air the room into the hallway to stop the sensor (worked for around 30 mins until in unit alarms started going off too).
