There is a rattle under the van.
There are many things that can cause a rattle under your vehicle. If the rattling occurs when driving and seems to happen more when accelerating or decelerating then it is more than likely a loose exhaust heat shield. A mechanic would typically get under the vehicle to see if any of them are loose. If the noise only occurs when going over bumps, it is typically caused by a bad suspension. Strut or shock. These are common failures as the van ages. A mechanic would typically drive the vehicle to verify the noise prior to looking underneath it. If you wanted to have this done, consider YourMechanic, as a certified mechanic can come and inspect the suspension of your vehicle.