The unsteady wall pressure on the aft deck of a multi-stream, planar supersonic nozzle is studied over a range of nozzle operating conditions corresponding to independent changes to the core and bypass stream pressure ratios. The data are processed using time-
frequency analysis and reveal various tones corresponding to transonic resonance as well
unsteady interactions of both separation and reflection shocks with the developing boundary layer. The position of the separation shock is shown to experience significant hysteresis effects, which subside at pressure ratios well above the design pressure ratio of the nozzle. Shadowgraphy images of the exhaust plume are also presented, which are then analyzed using the snapshot form of proper orthogonal decomposition. The findings from this low-dimensional analysis demonstrates how the first most energetic mode highlights the shock cell patterns whereas the second most energetic mode elucidates turbulence motions in the plume.