Its a 1992 TE300 4matic 158K miles---the engine has a click coming from the valve cover area that is fairly loud for about 6 sec. then it quits for 1 sec. then builds back up to loud for 6 sec. and repeats while idling. At higher RPMS the click seems to go away. I have fair mechanicing experience and would claim this to be a weak hydralic lifter, but I have assumed with over head cam there are none? More knowledge needed.?