Squeeze a penny through the neck of an uninflated balloon, so it rests in the balloon. Now inflate the balloon, and tie it off. Swirl the balloon so the penny rolls along the inside edge. Listen closely, and you'll here a slight hum. Now put a hex nut into a different balloon. Swirl it around... it screams!
When the penny rolls around the balloon, it rubs against the balloon a little, stretching it a little, making it vibrate a little. This makes a little noise. When the hex nut rolls around, it makes the balloon vibrate a lot, making a loud noise!