ok well here is the story, i have a 66 with a 289 in. when i bought the engine it was stock from another 66 that was rebuilt and had 15k on it so i stripped the smog, put in a performance manifold, 4brl. carb, headers, and full exaust. when i finished it souned very healthy...4 weeks later, i am now hearing a strange "clicking" sound, i thought mabey my header bolts were comming loose from expansion of heat so i tighted them up but i still heard the sound. i ran through the whole exaust and felt for leaks and there was none, so i lifed the car and tried to listen to where it was comming from, it sounded near my transfer case and near my right header but then i checked from the top and it sounded like its comming from the valve cover or near, so i dont know if im not getting oil to my lifters or what the problem is but please help me! 
I agree that it's likely a rocker arm.
