During boarding and taxing, passengers on the Airbus A320 may hear various aircraft noises, from the landing gear retracting to a distinctive ‘bark’ caused by the Power Transfer Unit (PTU). That ‘bark’ on the A320 (also heard on the A330) is caused by the aircraft’s Power Transfer Unit (PTU) undergoing self-tests to ensure hydraulic system redundancy. The bark is a distinctive sound heard by passengers flying on the Airbus A320 family aircraft but not on its rival, the Boeing 737.