I had the exact same problem! In my case it turned out to be a worn-out 'O'-ring on the strainer housing. Or it could be the strainer "cover/cap" is not screwed-on tight enough. There was no water leakage - only air being sucked-in. I would check this before proceeding with other troubleshooting.
Also, the 'O'-rings on the inlet & outlet hose fittings on the pump can also eventually "give-out" and cause a water leakage (been there/done that!) - so good idea to replace those every 5-6 years as easy preventive maintenance.
