Nozzle–Isolator co-flow configuration is recurrent in many hypersonic propulsion system configurations. The shock train dynamics generated in the isolator can lead to devastating effects if not understood properly. Shock trains in co-flow configuration, being less studied and not optically visualized in the past, made the prime motive of this work. High speed schlieren and wall static pressure measurements comprised the diagnostics of this work. Understanding of inherent shock train unsteadiness in co-flow configuration was attempted and the frequency spectrum of this unsteadiness is found to have predominantly low frequency content at around 20 Hz. This falls inline with the shock unsteadiness studied in the rectangular ducts with mechanical blockage. The perturbations emanating from downstream mixing layer could likely be the source of low frequency content. This could be analogous to the low frequency perturbations emanating from downstream diffuser in regular rectangular isolators as mentioned in the literature. Further analysis is required to probe into the details thoroughly.