[pontifex4]

Theory says that the two ways you can kill road noise in a car are by:

1.) blocking transmission of sound between the source and you
2.) reducing structure-born vibration

Most people who do this kind of work to make a living (and while I agree, I am not one) will tell you that you need to do both in the case of a car, since it is both a diaphragm which makes noise and an ineffective barrier for other noises.

There are variations on the theory of noise barriers, but, fundamentally, to block sound, you need to add mass. Popular mass loading materials include thick sheets of vinyl and even lead. Not foam.

Expanding foam (while it is really messy) will help you with the second class of noise: structure-born vibrations.

Believe it or not, another popular material for reducing structure-born vibration, as well as (to a lesser degree) for blocking sound, is non-hardening modeling clay. Seriously. Choose any colour you like. You'll find that jammed into crevices all over award-winning SQ cars at stereo competitions, especially in dash and door trim, and on the backs of thin plastic interior panels.

I should note that I haven't tried anything on my own car, yet, though I certainly intend to soon. Perhaps it's time for a detailed "noise killing in 944s" thread?

As AaronM notes, above, tire noise is the real culprit on these cars (you share the passenger compartment with the entirety of the rear wheel wells, so the largely undamped resonance from performance tires is transmitted to your ears quite effectively).