The design of hypersonic vehicles presents numerous technical challenges, with one of the most significant being the occurrence of unstart in scramjet engine inlets. Unstart refers to a disruption of supersonic flow within the inlet, resulting in thrust loss and potential performance degradation of the vehicle. In this paper, we conduct a numerical investigation into the unstart phenomenon in a rectangular hypersonic inlet operating at design Mach number 6 and lower design Mach number 5. The unstart process, induced by excessive heat release in the combustor, is simulated by adjusting the throttling ratio to analyze the internal flow structure of the inlet. This simulation includes the introduction of a plug in the combustor to emulate an increase in pressure and temperature. Our results offer insights into the complex flow physics associated with unstart in the present configuration, which features ramp angles of 10 and 21 degrees. We examine the effects of throttling ratio on the oscillatory behavior of shock trains, which give rise to two distinct phenomena: ‘little buzz’ and ‘big buzz’ in the flow. ‘Little buzz’ is characterized by high-amplitude oscillations of the separation bubble, while ‘big buzz’ involves violent oscillations of both internal and external flows. Pressure signals and contours illustrate the unsteady behavior of the separation bubble flow associated with the buzzing phenomenon across different throttling ratios.