My '73 1800ES recently developed a mysterious clicking noise. A couple friends agreed that it seemed to be associated with the valves. To test this theory I first checked and adjusted the valves clearance. It's only been a few months since the last time I did this and they were all still good or very close. Next removed the rocker arm shaft assembly and turn the engine over with the coil wire removed. The noise disappeared. When I did this pulled each push rod out to inspect it, non where bent or showed any unusual wear. I shined a flash light and inspected the lifters as best as I could, they all looked normal. Then I warmed the engine up and the noise became even more pronounced. Last, I checked my engines compression; it was an even, though lower than I hoped 130 psi warm. Any ideas about what the noise could be? I'm preparing to replace the cam, lifters, and pushrods will this solve the problem? If it will, any suggestions about what type of cam I should use to replace the existing one? thanks.