An experimental investigation into the flow-field of a modern jet nozzle has been carried out. Even in canonical flows (e.g. axisymmetric jets), turbulence and its consequences are not fully understood. For composite flows that are becoming more prevalent in the aerospace industry, the fluid mechanics have been studied even less sufficiently. The characteristics of a supersonic, complex, rectangular jet are therefore investigated via schlieren and PIV data. A large-window PIV system measures planar data, and a time-resolved schlieren instrument captures events of the flow at acquisition rates up to 400 kHz. As an initial investigation into this rich flow-field, the study focuses on probing the flow to find regions of interest, with particular attention given to identifying coherent structures, shear layers, and shock structures. Spectra of the data are computed and a dominant frequency of 34 kHz is universally identified throughout the experimental domain. Proper Orthogonal Decomposition (POD) is additionally used to extract coherent structures in the flow and to help explain where the unique signal may be coming from. Evidence is presented that suggest Kelvin-Helmholtz instabilities generated inside the nozzle may be forcing this high-frequency signal that is seen throughout the flow-field and in the far-field acoustics.