97 Rav4

My 97 Rav4 has a hard time getting oil pressure on initial start up after the engine has been shut off for a few hours. I tried installing an OEM filter but continue to hear the lifter noise and see the light on for 5/10 seconds or so. Is there a check valve somewhere that could be the culprit. I took the pan off but only have a sump and thats clean?? Help...