sometimes with an old worn down engine, the rod bearings can be replaced and that will extend the life of the engine a while. But, if the wear between the rod bearing and the crank journal is mostly on the side of the journal, no such luck. “Knocking” is a technical term, and means the rods or rod bearings are worn to the point where they knock against the crank journals instead of turning smoothly. Unfortunately many people with no special knowlege of engines will call any noise a knock and that makes it difficult for anyone who cant hear the noise to tell you what is wrong. I suspect, from the age of your van, that the crank journals are worn, and it needs to be turned, and oversize bearings installed to fix the the noise problem.