I also hear this noise when pulling away from being completely stopped. I don't know what it is, but I imagine it could be a returning rear brake spring - (the sound has the tone of a tweaked spring) or else a noise generated by some minimal rotation of the rear axle. My U joints and all rear bushings are fairly new. It could also be caused by sideways (?) movement of the axle. When starting from a stop, I'd think the torque from the engine loads up the rear and any play anywhere before the car moves.