My first thought would be that you exited the FSS so quickly that it was still playing its initialization sound effect and it got stuck for a bit.
My second thought is to wonder if you're near a signal source, as that sound isn't too dissimilar to the "garbled signals" sound effects you get from e.g. Degraded Signal sources.
My second thought is to wonder if you're near a signal source, as that sound isn't too dissimilar to the "garbled signals" sound effects you get from e.g. Degraded Signal sources.