In terms of CFD, we often analyze the stability of the error using Von-Neumann analysis, especially for FDM based problems. Should we follow the same approach for a compressible fluid flow using FVM ?
I would do a mesh refinement study (or change the element orders). If the oscillations are resolved and converge to a specific pattern, I would conclude they are not numerical errors. However, if they change randomly in each study, I would attribute them to numerical error.
You see, in closed volumes resonance phenomena may occur, which are related to instability inside the flow. The second reason may be the following: the step in the computational grid is very small, which leads to the fact that the difference between the flow parameters at the inlet of the computational cell and at its outlet is not too different: the difference in their value is comparable to the rounding error, the accuracy of the numerical method used. The solution in this case is to use a coarser grid.