98 lumina loud tick

After a 200 mile drive, the car had developed a loud tick that is in time with the engine speed. sounds like one of the valves making noises. I know that the oil has a lot to do with the valve lifters, but not quite sure what it does. I have checked the oil level and it's ok. The noise diminishes after the car get warm, about 4 miles of driving, but it doesn't go away. In the past I would hear the sound as the engine first started but after about 1 or 2 seconds, it would quit. this is the same noise but much louder.
Any suggestions of were to start?
Thanks Rex
One other item that might be helpful. the car has 180, 000 miles on it now.

My first guesses from personal experience would be to check the exhaust manifold closest to the firewall for a crack, and check for coolant loss on the passenger side of the motor which indicates it may have a lower intake gasket that is failing. Had both of these happen on my wife's 95 lumina.