11 month old rocking

Just wondering if this is normal... my daughter has been rocking forward and backward while sitting when music comes on since she was about 5 months old. She does the same thing though when she holds a toy, usually a ball and even when she hears loud sounds like the vacuum or a blender. Is this normal behavior? I took her to a playgroup yesterday and she was the only one sitting still, holding a toy and rocking.