You might be rubbing on the rear fender lining. It deforms every time the tyre hits as it is not a real stiff plastic. Have a look inside the wheel well where the rear bar meets the metal of the wheel arch. There is a bolt there that stick out a little bit and that is covered by the plastic fender liner. That area sticks out further then the rest of the liner and might be rubbing against the tyre. It will deform as the tyre keeps hitting it, reducing the amount of rubbing you might hear.
...or it might be just rubbing against the wheel arch itself.
