As i drive along there is a rumbling coming from the rear as if i were carrying a load of bricks in the back and they are rolling around.

Yet when stationary if i bounce the car up and down i dont hear any noises coming from the car like i did with an old car which needed suspension work,

What could it be? [:@]

 

with this much lil info to go on, id say either tires (cupped or thrown belt) or worst case a hub. look it up in wikipedia.