The acoustic characteristics of a liquid-propellant rocket engine, utilising high-test peroxide and ethanol as the bipropellant, were studied using numerical simulations with constant-frequency excitations. The simulated geometry included a porous catalyst, a retainer, a combustion chamber and a converging-diverging nozzle. Realistic profiles of flow parameters were imposed onto the internal flow volume, which was excited by a cloud of randomly phased monopole sources at frequencies up to 20 kHz. Acoustic modes were identified using an array of virtual microphones and maps of acoustic pressure. Several approaches for treatment of the flow in the diverging section of the nozzle were tested. Two types of boundary condition were implemented—a hard wall with perfect reflectance and a modal duct, the latter simulating the anechoic condition. They were applied firstly at the throat of the nozzle, and secondly at its outlet, with flow velocity in the diverging section clipped just below Mach 1. The results showed that the modal duct boundary condition provided the most accurate representation of the acoustic characteristics of the internal volume, and that the additional longitudinal modes induced by the hard wall condition interacted with other modes in the chamber. Furthermore, removing the diverging section of the nozzle from the simulations enabled more straightforward and accurate identification of the radial and tangential modes of the mostly axisymmetric geometry. Therefore, placing an anechoic boundary condition at the throat of the nozzle was found to be the most appropriate solution.