Oddly enough, I had the same problem.
I had a pair of tire defects from firestone. Both the rear tires did something I nor the mechanic from Firestone had ever seen before. The tread actually lifted in several parts of the tires. Normal tread, just some were higher than the rest. Not a wearing problem as if the higher parts were higher because the tread next to it was worn down, but literally, it was higher. Not a bubble either. It was only noticible while trying to balance the tire. The tire could be easily balanced but when you looked at it straight on from the side, you could see higher tread spin by. So when I drove the car at high speeds there was a constant vibration. Almost as if you were driving a small four cylinder not meant to be pushed up to 80 mph. But at lower speeds I guess the continental suspension smoothed the bumping out so it was barely noticible.