Once a baby chick gets through the membrane and begins to breathe the air in the air sac, oft times you can hear it start to peep and make chick noises. Once it "Pips" through the egg shell or makes a crack in it, then you can really hear the peeping. It goes on and off as it "zips" or turns in the shell until it knocks the 'fat end' of the egg off, and hatches.