1990 Toyota Corolla Engine noisy like knocking noise

Toyota 2E engine 1.3 1990 model. I did complete engine overhaul. 2 new pistons, new oil pump, rings, bearings, big ends, mains. When I started car it was smooth but then oil light came on then noise starts. I changed the oil to havoline 20w50 but same problem. Even at high Rev oil light still stays on but at times goes dim. Seems like noise coming from sump or crankcase. I took a valve that fits to the block in the sump area just by the oil strainer from another engine which had problems similar. I never clean the valve but it looked fine. I think it's called a release valve. Will that be the problem or something else. What is the purpose of that valve.