In this paper, we report a study of the noise due to unsteady transonic flow around an airfoil. The resulting unsteady shock waves on a blade create shock-related noise that can be studied computationally. We will examine the noise generated by blade-vortex interaction (BVI), an oscillating flap, and a thickeningthinning airfoil (Fig. 1). BVI is the aerodynamic interaction of a rotor blade with the trailing vortex system generated by preceding blades. It usually occurs during helicopter descent, or low speed maneuvers. Interactions generate the most significant noise when the vortex is exactly or nearly parallel to the blade. The flow can be initially modeled by two-dimensional unsteady transonic flow, since BVI is concentrated near the tips and the relative velocity can be assumed constant in this region. An oscillating flap is a typical configuration that is widely employed in the experimental and computational studies of unsteady transonic flow. Tijdeman (1977) used an oscillating flap to experimentally identify three types of unsteady shock motion. The thickening-thinning airfoil is another example used, because for symmetric airfoils, no noise signal is generated due to lift.