noise at valve cover

What is your question?

In the morning car starts okay. The engine sounds okay normal oil pressure as the car is running after 20 min oil pressure is dropping and I hear a rattling noise at the valve cover sounds like the rocker arms. I rev the car no rattling sound after a while the rattling sound is back. Changed oil and oil filter. The temp needle is at half. I cleaned radiator it is good no obstructions, water pump is working fan is working okay. The manual says 10w 30 oil and i put 40 oil. The valves are hydraulic, you thinks this is causing the problem