Experiments to investigate the fluid mechanics of whistling are reported. A model, consisting of a cylindrical cavity with rounded holes at each end, is used to simulate human whistling. It is found that the frequency is very near the Helmholtz resonator frequency, and that the resonator can be excited by flow through the smooth‐edged orifices bounding the resonant cavity. Furthermore, it is found that the flow velocity of the jet which excites the resonator must lie between limits that are proportional to frequency and that increase with both diameter and thickness of the orifice. It is concluded that whistling can be included in the same class of sound sources as the Rayleigh bird call and the Pfeifentöne, since the essential mechanism for exciting them depends on the instability of a jet to the formation of vortex rings and the interaction of the rings with a rigid boundary in the flow.