just got the son a nold 84 300sd and it has a knocking in thr valve cover area up front . well 300.000 miles i thouthe valve adj. pulled the cover and checked them and all were fine . started it up and there was no noise. looked in the valve cover and there was chain marks . installed new chain ,and only dropped it in the front cover once .fished it out with a magnet and luckily it didnt jump off of the pump. reinstalled the covet and it seems to be knocking again . any ideas