An analytical investigation is made of the sound produced when a line vortex translates over a parallel, nominally smooth rib-stiffener on a thin elastic plate. This is a canonical fluid-structure interaction that is believed to be an important source of boundary layer generated aircraft interior noise. To eliminate additional complications introduced by mean flow, the vortex motion is assumed to be controlled by its image in the plate. The arrival of the vortex at the rib is preceded by bending wave forerunners which are excited in the plate at that frequency V at which the flexural wave phase velocity is just equal to the vortex translational velocity n. These waves have a group velocity equal to 2n, and extend over a length of plate ahead of the vortex determined by the structural damping. The principal acoustic source is the interaction of this forerunner with the stiffener, and the sound is predominantly of frequency V. The amplitude of the radiation progressively increases to a maximum as the vortex approaches the rib, and subsequently decreases rapidly to zero with passage of the vortex over the rib. Numerical results for the efficiency of sound generation, its directivity, and the acoustic pressure signature, are given for clamped and simply supported conditions at the stiffener for aluminum plates in air.