If the pump and filter are above the water level, you probably have a leaky o-ring or gasket somewhere. It would most likely be the filter lid or the pump basket cover.
If you turn the multiport valve to closed after turning off the pump, do you still lose prime? If not, the problem is most likely at or after the filter. It could also be the multiport valve or any of the threaded connections before or after the filter.
If the filter is above pool level, the water above the central pipe may flow back to the skimmer if there is no check valve on the suction line. This would cause the "gurgling sound".