At first I thought the sound of a very light tapping was an exhaust leak so I changed out the exhaust manifold gaskets, I started the truck and still hear this sound. It sounds like its coming from the heads, I was wondering if is a common practice to re- rack the heads? Do they ever get loose? the sound doesnt sound like a rod knocking or anything just a lite tapping. I dont mind chaning out the valve cover gaskets and while im in there should I check this?