Got a new question now. Does the noise go away when you step on the brake pedal, or does it remain about the same until you get below 30 mpg, then gradually slow until you come to a stop?
If the noise goes away when you step on the brake pedal, I would suggest you make sure the anti-rattle clips on your brake pads are still intact and positioned properly. Also, inspect the dust shield between the brake rotor and steering knuckle to make sure it is not making contact with the rotor.
Can you estimate the frequency of the thump as you just start to accelerate as well as once you reach 30 mph? Is it once a second, once every 5 seconds, etc.? Actually, can you just tell us everything you can think of about the noise?
-Rod