Common cylinder operation is controlled by a pneumatic valve, when the need to open the pneumatic valve to control one end of the cylinder intake and exhaust at the other end, the need to close the pneumatic valve will control the other end of the original intake end exhaust, so every action has exhaust sound. Pneumatic valve is also a spring return is only one end of the intake to open the valve, closed only closed air exhaust by the side of the spring is responsible for closing. The action of this valve is one of the secondary exhaust sounds.
