A rattling from what sounds like the catalytic converter. It only happens as you slow down to a stop and then again when you accelerate from a stop. sounds like a part sliding back and forth on a bolt, but everything is tight?

does it get worse the faster you go? could be your lifters or valves ticking.

Check the heat shield around the catalytic converter

I would think the honeycomb ceramic "Brick" inside the Catalytic Converter has broken up, makes a terrible rattling when this happens.

It is most likely an exhaust heat shield or a catalytic converter that is breaing apart inside. Get underthe van when it has cooled off and "thump" the exhaust with your fist to see if you can locate the rattle. Often, the heat shield welds break where they are attached to the catalytic converter. I have fixed many of them by taking 2 or 3 large screw-type hose clamps and screwing them together to make them long enough to reach all the way around the catalytic converter and tighten them down to stop the rattle. If the noise sounds like it is coming from INSIDE the converter, you will have to replace the catalyst. (or ignore the noise)

this may mean your catalytic converter is broken and the rattle from the exhaust hanger that could have broke when the catalytic converter was struck by hitting a speed bump too hard or a curb.

probable exhause heat shield or maybe converter damage

Have the catalytic converter heat shield checked. Sometimes they vibrate loose and caus strange rattles or buzzing noises.

sounds like either starving for fuel or a clogged cat/ convertor

I was just wondering if it is the cadilac converter

as for the rattling sound have you checked the heat shield on the catalytic converter, mine was loose and was causing a rattling sound when it was at idle, and would go away as the rpm's increased.