Some guys forget or don't notice the soft plugs behind the thrust plate. If the plate were to loosen up you would loose oil pressure that way and it wouldn't take much movement of the plate to lose pressure. As for the knocking maybe the camshaft is bouncing off the plate. Take the pushrods out, take the distributor out, remove the intermediate shaft, take a long pry bar or screw driver and see if the cam moves fore and aft.