there's a good chance that the tire is already worn to the point that it has a "flat spot" on it and will not go away. Another possible and more rare occurrance could be a slipped belt. I basic terms it maens the tire is degrading within itself. Go to a tire shop to see if they can help or someone that deals with more of this type of problem to get better info then, if it turns into a warranty issue you can take the diagonosis to the dealer.. In writing of course.