High-frequency instabilities in liquid propellant rocket engines are experimentally investigated in a model scale research facility. Liquid oxygen and gaseous methane are injected in the combustion chamber at 0.9 MPa through three coaxial injectors vertically aligned. High-amplitude transverse pressure fluctuations are generated in the chamber at frequencies above 1 kHz by a rotating toothed wheel actuator which periodically blocks an auxiliary lateral nozzle. The chamber eigenmodes are identified in a first stage by examining the response of the system to a linear frequency sweep. In a second stage the chamber is excited at the frequency corresponding to the first transverse (1T) mode. The effect of the pressure mode on combustion is observed with intensified and high-speed cameras. Photo-multipliers and pressure sensors are also used to characterize the system behavior and examine phase relations between the corresponding signals. Flame structure modifications observed for specific injection conditions correspond to a strong coupling between acoustics and combustion which notably modifies the flow dynamics, augments the flame expansion rate and enhances heat transfer to the wall.