I used to be an engineer for a couple of heavy duty truck manufacturers and one of my projects was to improve the sound deadening. The very most important issue, which fortunately is the easiest and cheapest to improve, is to make sure that all cab openings are sealed. This seems so minor, but it the MOST important. Think of sound as if it was water and you are in a boat. The hull can be as thick as you want with as many layers covering it as you can add(just like sound deadening material in your truck), but if there is any hole in the hull, even a pin hole, the water will get in. So go thru and seal every crack and hole you can-windshield seals, door seals, seals around the pedals, holes in the firewall, seams around panels, wire and cable pass-thrus in the firewall, etc. Second most important is flat panel radiation and reflection. Any big metal panels will vibrate and create noise(drumming) so stiffen them. Also pad the areas so when sound does get into the cab, it gets absorbed by the paddings. This way, you hear the noise only once before it gets absorbed instead of having that same noise bounced around and around inside the cab. After that comes reflection and absorbtion. High frequency stuff(turbo whine) is easier to kill because it has short wavelengths and less energy. Padding with aluminum cladding works well because the aluminum reflects the sound and the padding absorbs the sound that gets thru the aluminum( the aluminum side faces the noise source). This works particularly well on the underside of the hood(cover the entire underside for best results), because a huge source of noise is noise coming off of the top of the hood and coming thru the windsheild. Low frequency stuff is the hardest to kill because it has long wavelengths and lots of energy(i.e., think driving next to a low rider with kicker speakers). This would typically be engine mechanical and combustion noise. Here you need thick, dense sound absorbers. Thick because the wavelength is long, dense because the power level. This would be something like 1/4 -1/2" thick reinforced rubber sheets(like recycled tires or truck mudflaps). This works best on the floorboards and firewall. Again, keep in mind that you want to seal everything so if you were to cut and fit sheets of rubber around the trans tunnel and firewall shapes, seal the seams with RTV or some sort of caulking. Putting a carpet over the heavy rubber would really help because you stop high frequency, low frequency, and reflected sound. Hope this helps.
