Results of an experimental investigation are presented that describe the characteristics of discrete sound generated by a constant area ejector that uses an underexpanded rectangular jet as the primary flow. The variation of screech tones, which are commonly observed in the spectrum of free shock‐containing jets with stagnation pressure and area ratio, show a staging behavior due to the presence of the ejector duct. Such a behavior is explained with the use of a feedback loop. In addition to screech tones, a different type of resonant oscillation within the duct is identified. These consist of acoustic resonances whose frequencies and mode shapes are related to the unsteady flow in the duct and the size of the duct. Two dominant symmetric (varicose) and antisymmetric (flapping) modes of the primary jet determine the nature of the ejector tones that are observed in the spectrum. Using a phase‐locked schlieren system, different longitudinal and transverse duct modes are photographed.