Before taking off the valve cover take a long  screw driver with the engine warmed up and running and place the tip againist the valve cover at about each valve spring with you ear againist the handle listen for the loose or bad lifter, you will hear the sounds of the other rotating parts of your motor but if it is a lifter or rocker  it will pernounced, do the same at the exh. ports and where the head pipe bolts to the manifold for exhaust leaks, it may be hard to tell by taking the cover off if the lifter is not on  the heel of the cam.