I have a question on what i could possibly do to get rid of the rattling in my e-150 van?

First thing to do its to find what is rattling.

You have two solutions. Either secure the two surfaces together to prevent them from rattling against each other or put something between the two surfaces.

I found that a good cheap material to use between two surfaces is foam carpet padding.

If a metal panel is vibrating I find the best solution is to stiffen it.

Alright cool thanks!

When we make your panels we will apply some of what edub said. That should help out quite a bit.

Best solutions for removing rattling are (in order of relative cost effectiveness)

Expanding foam between metal surfaces (cheap but high risk to vehicle damage if not done properly)

Foam padding between metal surfaces, and behind plastic panels

Sound deadenning products used in layers.

Some area's I would focus on in a van like that would be the roof, rear 1/4 panels and rear doors.