There may be several heat shields under your van, but typically the ones that cause a rattle are connected to the catalytic converter, a part of the exhaust system located approximately under the front seat on the passenger side. If you can, pull off the heat shield, as it is probably rusted at its mounting points and if you don't park in tall grass, you don't need it. If you can't, take it to a Muffler or general auto shop. They should remove it for little or no cost.
If you don't see a heat shield, or it is already gone, then hit the converter hard by hand (after the engine has cooled down). If it still rattles, then the converter is bad and needs to be replaced. Do it before those rattling pieces get into the muffler, or you'll be replacing the whole system.
