There are several places where sound dampening foam will work wonders for your car. For starters, if you drive an SUV or a wagon, try inserting a layer of foam at the bottom of any large bins back there. Try adding more in the rear lift gate, which may also resonate somewhat, especially at a higher speed. It makes a world of difference, as does installing foam under the hood (provided it’s fireproof, of course). Try also using some foam front kick flaps near the front doors and maybe in the A, B and C-pillars themselves. You would be surprised just how much difference it makes, as does the addition of a little sound-deadening material over the headliner.
For more info, see Sound Dampening Your Car