A valve for controlling the flow of fluid having first (16) and second (14) plates with offsetting apertures and a sidewall (12) disposed between the plates around the perimeter of the plates to form a cavity (15) in fluid communication with the apertures is disclosed. The valve further comprises a flap (17) disposed and moveable between the first and second plates and having apertures (22) substantially offset from the apertures (20) of one plate (16) and substantially aligned with the apertures (18) of the other plate (14). The flap is motivated between the two plates in response to a change in direction of the differential pressure of fluid across the valve.