The present paper describes experimental and numerical works to investigate noise phenomenon in supersonic flow that is discharged from a convergent-divergent nozzle. The noise phenomenon of flow is generated by an emission of 'transonic tone' that can be occurred independently of the general noise components at low pressure ratios that a shock wave arise within the divergent section of the nozzle. The results of obtaining show that the frequency of a transonic tone that differs from the frequency of a screech tone due to the shock-cell structures in a jet, and originates in the shock wave in the nozzle increases in proportion to the nozzle pressure ratio. The transonic tone of high-order has the directivity in the direction of the flow. Also the value of a predominant frequency of the wall static pressure fluctuation in the shock wave downstream is almost corresponding to the value of the frequency in which the transonic tone is shown.