The present paper reports on the wall-resolved Large-Eddy Simulation (LES) of a rectangular supersonic air inlet at Mach number 1.8. The peculiarity of the investigated configuration is the presence of a flat plate on which develops a turbulent boundary layer upstream of the first compression ramp. Two modes exhibiting low-frequency flow unsteadiness in the subcritical regime have been simulated: the little buzz (low amplitude oscillations) and the big buzz (large amplitude oscillations). A detailed analysis is performed on the spectral content of the flow and the pressure fluctuations in the duct in order to understand the underlying flow mechanisms. It is shown that the physical phenomena are similar in both cases with appearance of downstream-propagating expansion waves and upstream-propagating compression wave that are related to the terminal shock back-and-forth motion.