I'm working on the same question of how to reduce noise in my 400E.
It's best to combined a damping material (such as Dynamat) and a sound absorbing material, such as foam with a rubber backing, fiberglass or some still use jute. The damping materials reduces the resonant vibration which reduces the high resonant noises that enters into the car cabin. This is applied normally to metal structure of the car, and helps reduce the transferring vibration (noise) to other structures in your car.
The composite materials or the sound absorption materials are used over the damping materials. I personally selected a damping material with the adhesive backing, and where possible I'm going to use a self-adhesive composite sound absorbing material. One of the most interesting materials I found is a composite consisting of foam with a urethane facing that has lead sandwich between the foam layers.
If you decide to add sound materials, you may want to try to do all sections of your car (trunk, floor doors, etc.). If you do just your floor, you may notice more noise being transmitted from your doors, it sort of like opening a window.
When doing the floors or other areas cut the material in the largest section as you can and tape the seams, you'll want to avoid cracks were sound can enter. I believe most of the noise we are trying to eliminate in our Mercedes is road noise, so I would use some damping material and use more absorbing composite materials.
I notice on my car a lot of this noise comes from the door area, door post, rocker panels (as mentioned, with good corrective suggestions), this is a difficult area to add any sound absorption material to. I'm going to remove a door panel and try various material thickness to see what fits best, if you are going to do your doors, I'll let you how thick of material I used.
All sound materials usually work, the thickness is important for sound and fit, cover all areas possible, tape seams, fill all cracks.
Enjoy the quietness, hard to find today.
500E sway bars
210 16in wheels