sound insulator

you could use the spray stuff,Sound Killer, or use dynamat or other similiar products.A cheap way of doing it also is using self adhesive floor tile,I know it sounds stange,but it works and its a lot cheaper.

I just bought a length of rubber mat from the hardware store to go under the carpet. Just for a little insulation. But when I started my car it was significantly quieter. Now I think I'll install a stereo.

67stang7,
The stuff you're refering to may also be called Peel 'n' Seal. Those products are asphalt based building materials usually with a foil facing and a constuction grade adhesive. Up until a few years ago the products for cars were not much better than using these products. In fact, some companies repackage this stuff as their entry-level deadening product.

What these products and many of the newly developed ones do is called 'mass loading.' Essentially, by applying these products to body panels you are changing the mass of the panel and therefore changing it's ability to tranfer acoustic energy through the panel. Vibrations, at certain frequencies may be deadened and result in a quieter ride. Using deadening products in addition to absorbing products will yield the best results.

I personally would recommend using products designed for the automotive industry. The adhesives used are far better. PnS is infamous for falling off car roofs and door panels. Secondly, several manufacturers have developed better, more efficient compounds. The new products are not asphalt based which can smell when it heats up. Dynamat is the most well-known and most expensive. IMO you're payng for their name. Try looking at companies like Second Skin Audio, B-Quiet, or eDead. They all make decent products at much more affordable prices.

I used this stuff, http://www.quietcoat.com/index.html

You can see phots of what it looks like on page 5 of my Cardomain page. I am very impressed with the results.

http://www.cardomain.com/ride/597572

Can you spread a rust inhibitor along the floors and wheel wells and then also spray on a polymer like Quiet Car? Or do the two not mix? I would like to rust proof mine as best as I can but also soundproof it.

I used the peel n seal stuff from lowes. Worked fine for me. It seems to be holding up pretty well too, however on my floor I have some matting down on top of it as well. The strips I put on the door seem to be holding up OK as well.

http://www.pro-touring.com/forum_main.htm

This thread has a lot of good info on it, check it out.