Thermoacoustic instabilities are recurrent in ramjets and may cause thermal and mechanical fatigue, as well as engine extinction. To study and predict the development of such instabilities resulting from the coupling between pressure and heat release rate fluctuations, the internal acoustic field itself needs to be characterized and predicted. As a first step, a description of the acoustic field in a two-inlet sidedump ramjet in a liquid-fuelled configuration is proposed relying on numerical simulations. Two predominant oscillation modes are identified by spectral analysis. Their origin and spatial structure are investigated comparing the numerical results with a linear acoustics reduced-order model for pressure fluctuations. Lastly, complex phenomena observed by numerical simulation are studied by an ad hoc correction of the linear acoustic model introducing a harmonic translation velocity to the mode shape, replicating the trends observed numerically.