Anyhow, since I had most of the panels out, I decided to take the time and replace the old fiberglass insulation with something new and add some sound dampening as well. I did a little research and decided to use a knock-off dampening material similar to Dynamat or Fatmat. Luckily, a good friend of mine had a bunch left over from doing his van and it cost me $0. It's a peel and stick application and it helps keep the panels from vibrating and gives the panels a bit more thickness to help keep the resonance down. I put pieces where my hands could fit: doors, firewall, side panels, etc. I put more detail into the footwell area with both the dampening and insulation to keep our feet a bit cooler when driving on hot days as well as wind noise. I also added some of the dampening material behind the front grill on the exterior and then sprayed over it with a rubberized undercoating to keep down wind and road noise.
Then using some peel and stick foil type insulation from Home Depot,
I notice a big difference in the way the van sounds and feels on the inside. When It been sitting in the sun for a while, I don't feel like I'm sitting in a steel box. Furthermore, the sound dampening material really make the doors sound solid upon closure. Pretty easy, cheap and satisfying project. If you haven't taken the time to pull your interior panels and inspect your "shell", I highly recommend doing so... You never know what might be lurking in there.