The aim of this work is to investigate the sound produced by a Helmholtz resonator excited by a boundary layer. A deep cylindrical cavity with a small rectangular opening was designed to allow Helmholtz resonance as well as longitudinal and azimuthal acoustic modes within the cavity to be excited by varying the wind tunnel flow speed. Experiments performed show how lock-on between each of these three acoustic resonances and the shear layer hydrodynamic modes can been generated. The pressure measured by means of a microphone flush mounted with the internal surface of the cavity wall is used to phase-average the planar Particle Image Velocimetry (PIV) data in the shear layer region. The acoustic energy generated is calculated by applying the vortex sound theory developed by Powell and modified by Howe for its application to wall bounded flows. Practically, this can be achieved by extracting the flow velocity and vorticity the from the PIV data and by computing numerically the acoustic particle velocity field. The acoustic sources are localised in space and quantified over an acoustic period providing insight into the sound production of flow-excited cylindrical cavities.