[magicoolbus]

Look into a product called Mass Loaded Vinyl. Spendy, but very effective at noise attenuation if installed with a 'barrier.'

I did a combination of things in my burb:

I used a roofing product called, peel and seal products on the roof and floors-took 4 or 5 rolls IIRC.

On the side panels, I used a DIY lizard skin by mixing elastomeric roof compound and 'micro spheres.'

Lastly, I added a 1/8 MLV to the floor.

All together, I would estimate nearly 400 bucks into the project--well worth the effort. This does not include the cost of the headliner and the carpet kit.