Experimental and numerical investigations of an ultrasonic whistle are presented. The whistle design follows Beeken . It consists of two resonating cavities and a small exit pipe that can double as a resonator. In the experiments, the whistle is powered by a regulated high-pressure air supply. The whistle is tested over a wide range of supply pressures. Farfield acoustic measurements are made in a fully anechoic chamber in the frequency range 0−50 kHz. The noise measurements are supplemented with two-dimensional unsteady Reynolds-averaged Navier-Stokes (uRANS) simulations to investigate the mechanisms of ultrasound generation in the whistle. Three distinct operating regimes are identified in the simulations - (1) low-pressure regime in which the two cavities operate out-of-phase and Helmholtz resonance determines the whistle frequency, (2) transition regime, where several tones are observed and multiple resonance mechanisms are simultaneously at play, and (3) high-pressure regime where the two cavities operate in-phase. The whistling mechanism in the high-pressure regime remains unclear, but it appears that the cavities do not play a role in sound generation in that regime. In the experiments, the behavior of the whistle is similar to that in the simulations in the low-pressure and high-pressure regimes, but the transition regime is not observed.