The physical mechanisms for the occurrence of high-amplitude, high-frequency tonal noise generated by the flow through the gap between the slat and the main element during aircraft approach are explored using a detailed local stability analysis of the slat wake. Based on the physical characteristics of the wake, vortex shedding frequencies are predicted for different slat deployment angles. It is proposed that the observed tonal noise is due to a whistling mechanism. When the vortex shedding frequency at the slat trailing edge matches one of the transverse resonance frequencies of the gap between the slat and the main element, an intense, tonal sound field is produced. The predictions of tones and frequencies from the proposed whistling mechanism are in good agreement with experimental measurements.