440 head noise. Perpetual problems. Problem solved

There are no adjusters on the rocker arms. They look stock. I took all the push rods out and they are all straight. Nothing looks broken or cracked or out of place. I plan on draining a couple quarts of the oil out and replacing it with thicker. And I have had some on going problems like loose stearing and cooling problems and vaccum leaks but you guys helped me fix them all. This is the only one that I can't figure out. I have checked everything I been told to check and everything I can think of. The sound sounds bad and it's coming on the passenger side valve cover area. When I started it today after it completely cooled down it didn't make the noise at all until after about a couple minutes then it progressively got louder. I'm not real sure what to check next. The car sounds great out the tail pipe and has tons of power and isn't smoking but I don't know what damage I might be doing.