I always what exactly that noise was, in High School I could here that halfway down the hall in a concrete building.  It drove me up the wall.  I would enter the classroom and ask the teacher to turn of the monitor, when they would ask why and I would tell him the rest of the students strained and could hear it.  A lot of people thought I was hearing things or it was birds.  Also the noise you played as the monitors was way off, not sure if you made it deeper for others to hear or not, or just the way the audio picked it up.