A numerical model employing the vortex method is used to investigate the separated flow around a trip rod placed upstream of a resonator tube. The acoustic power generated by the flow is calculated using Howe’s theory of aerodynamic sound. When the trip rod is placed far enough upstream, natural vortex shedding from the rod proceeds and no net acoustic energy of the tube resonant mode is generated. However, when the rod is placed close to the tube, the vortex shedding becomes locked to the acoustic field, changes to a symmetric mode, and the flow generates net resonant acoustic energy per acoustic cycle. These results are consistent with previous experimental observations and provide insight into the flow structures responsible for the transfer of energy from the flowfield to the resonant acoustic field.