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