Sound generation by a grazing flow over a cavity without and with a grid of louvers in the opening is investigated. Simple vortex models are presented that describe the hydrodynamic resonance condition of self-sustained flow oscillations for rigid and elastic louvers. Calculations show that in both cases resonance can occur in distinct intervals of the Strouhal number based on the louver distance. The sound power radiated by elastic louvers is analysed. It is shown that flow-induced vibration of the louvers can be eliminated by increasing louver thickness.