Reducing the interior noise level in your vehicle will make your journeys more enjoyable, leave you less stressed and prevent you from having to shout when trying to talk with your passengers. You'll also be able to hear your stereo better, especially the bass notes. Apart from the engine, the two main sources of noise inside a vehicle are the sound of the tires on the road surface and the sound of the air rushing past the car, especially when you are driving close to other vehicles.
Remove the seats from your car. It's actually easy to do with a good wrench. When you've done this, lift out the carpet from the vehicle.
Place vehicle insulation liners on the floor of your vehicle. These are usually made of foam or tar material. They are deigned to absorb sound, especially tire noise coming from under the vehicle.
Use a wallpaper brush or roller to press the insulation liners down into the floor. Don't forget the boot as well. Place the carpet back on top once you've finished placing the sound-deadening liners.
Use the opportunity while the seats are out to soundproof the doors as well. Use a can of sound-insulating foam that's specially designed to expand into the hollow space inside the car doors.
Remove the interior door panels. A broad, flat screwdriver will usually be enough to remove the plastic pins holding the panels in place.
Spray the foam into the hollow spaces, being careful not to spray the windows. The foam will expand dramatically into these spaces, so you won't need to spray too much.
Wait for the foam to dry and then replace the door panels.
Don't forget the fender wells. Use the tar liners (or the foam spray) to deaden sound from these areas.
Remove any dirt from inside the fender wells and apply your chosen liner to the inside. Its adhesive side should be in contact with the metal.
Replace the seats.