To study the tonal noise characteristics of the leading-edge slat of high-lift configurations with and without a deployed trailing-edge flap, experiments are conducted in the D5 aero-acoustic wind tunnel at Beihang University. The numerical simulation method is used to obtain the necessary flow information. The experimental results show that low to mid frequency tonal noise generated by self-excited oscillation within the slat cove is dominant and its corresponding frequencies are basically unchanged whether the flap is deployed or not. However, the primary mode of self-excited oscillation within the slat cove switches to a higher one when the flap is deployed. Further analysis results demonstrate that variation of the primary mode is found to be closely related to the flow characteristics in the self-excited oscillation feedback loop. The number of the primary mode is generally proportional to the ratio between the vortex shedding frequency and the self-excited oscillation frequency. The flap being deployed results in an increase in the effective angle of attack of both the main wing and slat, which leads to a thinner separating boundary layer, thus increasing further the vortex shedding frequency and this ratio.