This sounds more like an exhaust problem than anything to do with internal
engine components. Check the exhaust flanges where they meet the cylinder
heads. Is there a bolt or nut loose? Is there rust at that joint? Also--I
had this happen--the O2 sensor can fall out of it's hole, making the same
exhaust sound. This has nothing to do with ignition wires,  and oil
additives won't help, either. My guess is that there's something loose in
your exhaust system and when you stop the van  or mess with something else
up top, it goes back together for a while. Hard to tell without seeing it,
of course.