ENGINE CLICKING NOISE, HELP???!!!?

I have a 1994 Camaro with a 3.4 liter V6. Since i've owned the car i've noticed a clicking noise coming from the engine that clicks consistently with the rpms. I believe it's either something to do with the valves or maybe exhaust. When i change the oil i add the engine restore additive and the clicking stops for about the first 20 minutes after the oil change and then is back to stay. What is this noise??? and how can i repair it??