Choked flows are common in microscale combustor, impinging supersonic microjets for concentrated cooling/cleaning of electronic components etc. Plain channels of constant area section being the simplest are frequently employed in these micro-fluidic systems. In such cases, it is common for the pressure fluctuations at downstream to interact with the choked flows through the micro-devices with undesirable consequences. In extreme cases, the amplitude of an oscillation may be so large as to cause the microchannel to become unchoked momentarily. It is therefore important to understand the response of a choked microchannel to excitation by a large amplitude downstream disturbance. Previous studies appear to assume low amplitude excitation, shock-free flow or ignore possible asymmetry in thermal and flow fields. Hence, three-dimensional, transient numerical simulations based on Navier-Stokes-Fourier Equation (NSF) were performed to predict the temporal changes in thermal and flow fields of pressure-driven flow in a constant area straight microchannel, as the downstream disturbance dies out exponentially. The walls of the channels are isothermally heated at 343K. The solutions presented here exhibit very interesting phenomena such as the existence of multiple supersonic pockets of flow inside a straight microchannel with subsonic inlet condition, flow evolution from choked to unchoked condition and then again back to choked state as the disturbance propagated upstream from the exit.