Think about bubbles in a bottle of carbonated soda. Before you open the bottle, the carbonation is a liquid inside of the water because of the pressure inside

Think about bubbles in a bottle of carbonated soda. Before you open the bottle, the carbonation is a liquid inside of the water because of the pressure inside the bottle. When you open the bottle, the pressure drops and the carbonation converts to a gas and becomes bubbles. But, imagine if instead of opening the bottle, you just somehow reduced the pressure then quickly increased it back to what it originally was. The carbonation would go from liquid -> gas -> liquid very quickly. CO2 bubbles would form then collapse. Converting from liquid to gas is not a special property of CO2. There are liquids in your joints that will convert to gas if you drop the pressure. That's what happens for an instant when you crack your knuckles. What you are hearing is the bubbles of gas collapsing as the gas converts back to liquid.