I just got a new Littman Cardiology III stethoscope as a birthday gift, and I noticed something strange. The pediatric side has a small pinhole in the center of the diaphragm. Is this normal? I am just a pre-med and have no idea how to use this thing, but I would like to know if I should send it back. I can hear things if I use the adult side, but the pediatric side doesn't seem to do anything.