You could also have a broken steel belt in one of your tires. the steel will eventually show up on the outside of your tires it will look like little metal hairs sticking out of your tire...you will have to run your hand lightly over ur tires or get down on your knees and try to see if you can see them sticking out.
I've had this happens twice over the years,it was next to the underbody not on the side that you can easily see
