The Maryland Centrifugal eXperiment (MCX) is a novel rotating mirror plasma, which is axially confined by the centrifugal force. The interchange modes can be stabilized by the shear in the azimuthal rotation velocity. Both these goals can be accomplished if the rotation speeds are supersonic with a Mach number similar to 5. So far Mach numbers similar to 2 have been accomplished. Thus residual interchange fluctuations are observed on sixteen magnetic pick-up coils placed azimuthally at an axial location mid-way between the central section and the mirror throat. The fluctuations are analyzed and modeled using a low-dimensional system of equations for the interchange mode coupled to zonal flows.