This means the tire tread is starting to separate from the tire. It develops when air gets between the inner liner of the tire and the tread. This can be caused by improper flat repairs, hitting huge pot holes in the road and problems during manufacture.
Symptoms include: your vehicle feels like it’s doing the hula. The faster you go, the worse it gets. But sometimes you’ll only feel a slight vibration if the separation is just starting and the tire is on the rear.
This was the case in the video below.