So I went out on Google with keywords like “road noise” and “sound deadening”. Introduction to Car Audio: How to Tame That Road Noise mentions materials like B-Quiet that can be installed to damp out noise. Dynamat Door Kits are another useful goodie. Quiet In There actually goes into how the sound gets into the vehicle in the first place, and has a number of links to useful products. How to Sound-Deaden Your Car looks handy.
Have any of you had experience with this?