What is the cost of header repair?

A better explanation wuold be good if I knew what a header was? Or what is causing the clicking! I don't know, that is why I'm asking. I am not a mechanic nor am I mechanicaly inclined, I don't want to take it in, not having a clue and being taken advantage of.Can you explain what it is. The van has 150,000 miles, has had maintained oil changes, it will click for a while, then stop and start all over again.