The characteristics of self-sustained oscillations are investigated with high-resolution implicit large-eddy simulations of supersonic turbulent flow (M∞=2.0) past a three-dimensional rectangular cavity with length-to-depth ratio of 2. First, the mechanism driving the self-sustained oscillations is verified to be a feedback-loop mechanism between the shear-layer instability and acoustics disturbances. The noise source in located near the trailing edge, and the generation of feedback compression waves is mainly related to the passage of vortices over the trailing edge. Second, the effects of the upstream boundary-layer thickness are analyzed. The upstream boundary-layer thickness has significant impacts on the features of noise radiation. As the upstream boundary-layer becomes thicker, the dominant mode of cavity tones is varied to a lower frequency, and a 8 dB decrease in sound pressure level is observed in the broadband noise radiation.