Watch any car movie from the 1970s or earlier, and you'll notice something strange about the dialog: the actors spend most of the movie screaming to be heard over the engine. Starting in about the 1980s, automakers began putting heavy emphasis on making cars quieter and more civil by installing intake and exhaust resonators and mufflers. But making the drivetrain silent had a strange side effect: instead of screaming to be heard over the engine, drivers now sat in quiet annoyance, trying to ignore all of the million other little squeaks, groans and booms that engines had once drowned out.
As a whole, the 2010 Civic and cars of its generation weren't particularly loud cars. Honda's gotten so good at controlling engine noise that the car as a whole was quieter, but the sounds that remained were unnatural and annoying. Even that secondary source of noise, rushing wind running over the car, had been pushed to the background, owing to the car's advanced aerodynamic design and minimal air disturbance. But even then, a certain amount of engine noise and vibration will overcome the remaining road and chassis noise -- unless, of course, the engine isn't running. This is certainly the case with the Civic Hybrid, which can run so silently that you can easily hear every grain of sand as it pings off the bottom of the car.
Tires typically account for the majority of road noise in a modern car. A very regular tire tread pattern like that found on the DX's original-equipment Dunlops will create a sort of "harmonic" that increases road noise. This, combined with shorter and stiffer tire sidewalls, contributes to increased road noise. Tire Rack rates this tire's road noise at an unimpressive 6.3 out of 10, and the Civic Si's performance tire at a slightly better 6.6 out of 10. Consider swapping to a quieter tire, like the Continental Procontact -- 8 out of 10 -- or the ExtremeContact DWS, which rates 8.4 out of 10. Just shop around, and find a tire that meets your noise needs and has the performance credentials you're after.
The Civic as a whole is a fairly aerodynamic car, but there's room for improvement when it comes to reducing wind noise further. The Civic's door mirrors are a potential source of wind noise; air flowing off the edge of the hood and around the windshield goes right into the little channel between the top edge of the mirror and the door, going into the weatherstripping around the window. Consider a thinner "aero-style" mirror, like those sold by Mugen for the Si. Under the car, you might consider installing a full-length, sheetmetal "belly pan" that covers the entire underside of the car. A belly pan will smooth airflow under the car, which is low enough to the road that any sound beneath ends up bouncing back and booming off of the underside of the floor. You'll even pick up a bit more high-speed stability, and could see an increase in fuel economy.
Soundproofing materials work best when they create an unbroken "bubble" around the listener. Mass-loaded vinyl mat is easy to apply and cheap enough that you could apply it over most surfaces inside the car. If you're using a belly pan, you can take double-advantage of it by covering the top with soundproofing material like MLV, Dynamat or a cheaper equivalent. That's going to go a long way toward reducing noise. You can take this a step further by applying foil-covered and heat-resistant soundproofing to the underside of your hood, and inside the front trunk wall, under the carpeting. Removing the door panels and sticking insulation to the inside of the exterior door sheetmetal are next, and insulation along the firewall behind the dash will make a big difference. If nothing else, you should seriously consider doing the doors, removing the plastic inner fender liners, and sticking soundproofing mat to the tops of them.
