I would first suspect you have leak somewhere on the equipment pad. If you dont have any noticeable wet spots on the pad to start investigating, try this. Get the unit primed and running, bleed all air off and let it run 10-15 mins then shut the pump off and see whether you can hear the air going into the system and water draining back through the suction pipes to the pool.
Closely listen to, check and snug up: the air bleeder valve on the filter, perhaps the gauge, then the unions in/out on the filter and top of the multiport valve, the filter drain plug. Then move to the pump - strainer basket lid (lube the o-ring well), drain plug. Then move to any jandy valves on the incoming lines or return routing.
You likely have something that is tight under pressure and loose when idle. I had a confounding similar experience with a union on my unused tab chlorinator at startup this year. Loose! but held water under pressure. I ran the unit 24/7 for a couple of days to keep it primed until I traced it down.