It comes from underneath the shifter, and you can feel it in the pedals. it's not an axle or strut problem because they replaced those and the thumping was still there. They don't think it's a transmission problem. The thumping never changes in loudness, speed (thump, thump, thump never changes in distance between thumps), it starts right from when the car starts moving, the direction of movement doesn't matter, and the thumps continue until the car comes to a complete stop. Putting it in manual shifting doesn't change it and it's the same in automatic shifting mode.

I can't diagnose your sound, but here is a place that might help. See if you can find the same noise, and if so, you might find out what is going on. Good luck!

http://cartalk.com/content/features/Noises/

Maybe someone misplaced a subwoofer that is powered by the rotation of your tires??

Let the guy out of the trunk then take the car to a shop.

Check the steering rack, commom problem on these cars. Have someone turn the steering wheel while you watch the inner ends of the tierods for verticle movement.