Hush - S4.E18

A babysitter is called in to take care of a sick boy. As a game, the two make up a pretend creature out of household parts, one that eats sounds. The creature comes to life, sucking the sound and the life out of everything in the house. The two try to evade it, but the boy's uncontrollable coughing gives him away. The creature then becomes sensitive enough to the girl's heartbeat to find and kill her too, then goes out into the world for more "food"".