Abstract: The present invention provides an anti-whistling duckbill check valve for use in a fluid flow path for permitting relatively free flow of fluid in the path in a first direction from a first to a second end of the valve and for preventing fluid flow in the path in a second opposite direction. The check valve includes first and second lips extending in converging relationship to each other from a base portion of the valve. A vibration dampening rib is located on each of the lips wherein the ribs are configured to have a natural frequency substantially equal to the lips. The ribs vibrate out of phase with the lips such that the vibration of the lips is dampened.