Under certain operating conditions, a high power, multifilament ion source can effectively function as a spontaneously excited ion‐acoustic resonator. The fluctuations, initially excited by a current‐driven instability at the thermionic source cathode, exhibit a Lorentzian spectral profile where the resonant frequency is solely determined by the spatial cathode configuration, and the resonance half‐width is set by the mean plasma ion lifetime. The ion wave‐cathode interaction, responsible for the resonant behavior, entails a feedback mechanism dependent on the electrostatic wave scattering properties of thermionic filaments operated under space‐charge‐limited conditions. Fluctuation amplitude levels are insufficient to significantly perturb the ions and consequently, the instability does not affect the performance of a multifilament source neutral beam system.