To say that modern cars are quieter than older ones is both an understatement and an oversimplification. There's a world of difference between cars produced during the late-1990s and those made during the early 1980s; part of this difference in feel has to do with a focus on increasing chassis rigidity, but much of it comes down to the more modern car's advanced sound control. You might be surprised how far a few cheap soundproofing tricks will go in changing the character and livability of that rattle-trap you call a project car.
Remove your vehicle's door or trunk interior panels, measure the respective piece and cut three pieces of stage curtain sized to the opening. Lay the stage curtain on a table, stack the pieces on top of each other and staple just the edges of the material every three inches. Put one staple in the middle just to keep the pack from sagging. Lay a bead of adhesive down all over the inside of your panel in closely spaced, horizontal S-shapes and press the material pack into it. The layers of curtain will absorb much of the sound energy that tries to make it through. This may be one of the more expensive approaches, but the stage curtain's flexibility and mass will offer excellent sound control.
Perform the same procedure, but this time using a single layer of egg-crate-style foam mattress bed pads. If you don't happen to have an old one lying around, check either online, at a hospital supply distributor or at your local discount mattress outlet -- preferably one that sells used mattresses. Odds are good that they'll have some old mattress toppers laying around, and probably in the dumpster, since most customers will just buy new ones. You may want to consider handling the used pads with gloves, at least until you can get them disinfected. While not quite as good as true acoustic foam, these pads will deliver 90 percent of the former's acoustic performance at a fraction of its cost.
Spray panels with limited interior space with 2 to 3 coats of a polyurethane truck bed coating. Solid polyurethane itself isn't a particularly good sound deadener (it lacks the elasticity to fully absorb sound energy), it will add mass to your panels and modify the panel's harmonic resonance frequency. Heavier panels require more input energy to vibrate, which will reduce transference of all but the deepest and most powerful frequencies. All materials vibrate at a certain resonant frequency, like a wine glass at a high-C; bonding together two materials with vastly different resonant frequencies will go a long way toward reducing panel vibration and noise.
Drill a small hole in the top of your panel and inject liquid insulating polyurethane foam. Often used in residential applications, this fairly inexpensive foam goes in as a thick liquid, settles to the bottom of a cavity and then expands to fill it. Foam has a very high insulating value, and does at least as good a job of absorbing sound as any aftermarket soundproofing mat. This is an ideal solution for A-pillars, C-pillars, kick panels and other hard-to-get-to cavities in the car that may act as a resonating chamber for sound. The only real drawbacks to using foam injection are that the foam will go anywhere that a liquid would, and that the solid foam will encase any wiring or tubing inside the panel.
