Where/how to add sound dampening?
 

I recently picked up a 2008 E-350 extended chassis van, and the road noise is something awful at highway speeds. Just a constant roar and white noise from everywhere.

What are the main sources of noise in these vans, and have people had any success in adding some sound absorbing materials in? Where are the best places to try putting said materials to have maximum effect for the front seats?

Mine have never seemed that loud on the road.
With this one, I did put in some sections of the sticky black mat sound deadner on the spots where the window openings are. (mines a cargo van)
That got rid of much of the tinny sound in the van.
I replaced the padded floor mat with wood and foam board insulation.
In the seat and doghouse area I installed some foil backed ridgid foam, all seams raped with foil tape.
I have also cramed all the openings between the inner and outer body panels with recycled plastic grocery bags.
I put plastic vapor barrier over all this and pretty much sealed all the small opening, creating a pretty good vapor barrier.
I have also scored a pretty complete plastic factory wall kit to cover it all.
The bigest change came with the black mat.

I've done four different E-Series cargo vans where insulation and other sound deadening has been added. I find placing 3'4" plywood on the rear area floor makes a world of difference. Sound killing mat on the side walls and roof is very helpful too even if its a single layer.

I've used unfaced fiberglass insulation in the lower side cavities and filled the door shells too---making the whole van much more quiet. If you're able to add a sliding curtain just behind the front seats you'll find that too is helpful. An added benefit there is the heater and A/C are much more efficient, creature comfort greatly improved.

On the side walls I've used radiant barrier insulation, the type with foil on one side and PVC sheeting on the other, foil side spray glued to the body sides. This has another benefit of significantly reducing condensation that's known to accumulate if you're in a four season climate.

And big tip---if the original factory floor mat is still in place remove it and inspect the floor for signs of rust. Those mats absorb moisture in the spun fabric backing and never evaporates back out creating rust that goes unseen until rust has formed.

I'm with JWA on the 3/4" ply.
I have that down in the rear half of the van, where storagwe is/will be. In the front half it's 1/2" hard board foam with 1/4" underlaymant over that. All seams taped.
That mad a huge reduction in noise now that I think of it.
I was going to use fiberglass in the walls, but wanted to experament plus I'm cheap!

I have a '92 passenger and it seems to be okay quiet. Of course I'm coming from my work van with tools and parts rattling around so it may just 'seem' quieter. How is your exhaust? Roll down a window and drive by a big sound barrier/wall and see if your tires are noisy. Like they said it never hurts to pull up carpet to at the least check for rust in the front floor boards. That factory padding is like a sponge. Rusted holes in my '88 and I never knew until I took up the rubber mats. I have very little if any padding under my carpet in my '92. Thinking about something like a laminate with padding on the floor. Not sure how well it would work in a vehicle and how bad things would slide on the floor. Check out some custom van forums for ideas and van camping is a great 4 season resource.

Not in Stayhound, my '98 E150 but I've used Dynamat in the lock box in the back of my CJ5 and one thing to bear in mind when using this or similar products is to use alcohol wipes prior to installation.