noisy intake and exhaust valves?

my problem is that my cars engine makes a loud clicking noise (valves or tappets) it doesn't make this noise when i start if from cold but when the engine warms up it makes a loud clicking noise that comes and goes it stops making this noise when i rev the motor past 3500 rpms i recently changed the oil and filter i used 30 wt oil and fram filter i was told it was the oil pump going bad or the lifters are getting stuck i removed the valve cover and noticed that the intake valves have hydraulic lash adjusters but the exhaust valves have manually adjustable ones how do i fix this problem it gets very loud sometimes and im afraid to ruin my engine besides the noisy valves it runs good