Abstract: Flow past an open cavity has been studied extensively, while less attention has been paid to the effects of confinement due to side walls, which produces rich flow dynamics and noise characteristics. In this study, the effects of confinement on flow structures and noise radiation in three-dimensional rectangular cavities are investigated experimentally. The length and depth are fixed, and five ratios of width/length （W/L=0.1–0.5） are considered. The measurements are performed in an acoustic wind tunnel. The pressure oscillations are onset after the wind speed is greater than Ma 0.03. Once the wind speed is greater than or equal to Ma 0.20, the flow and noise radiation are dominated by the self-sustained oscillations corresponding to the third Rossiter’s mode. Furthermore, the present experiments show that the local pressure oscillations and noise radiation of this frequency can be weakened or even eliminated when W/L is equal to or less than 0.3 for the wind speeds of Ma 0.20 and Ma 0.25. The upstream OASPLs in the far field can be reduced by more than 3 dB when W/L decreases from 0.4 to 0.3 at Ma 0.20. By analyzing the surface pressure and TR-PIV（Time-Resolved Particle Image Velocimetry） results, it is found that the suppression of the tonal noise is closely related with the changes of the primary recirculation and some secondary vortical structures by decreasing W/L. In particular, the intensity of the primary recirculation is greatly weakened with strong confinement effects, and the feedback process is not strong enough to produce self-sustained oscillations.