I don't know why it wouldn't be covered under warranty. It sounds like you may have a thickness variation in the rotor that is caused by something causing run-out in the rotor. GM dealers have an extensive TSB and special tools to repair this condition. If the car is stock and hasn't been improperly serviced by someone, that is what the warranty is for.
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Saturday, March 29th, 2008 AT 12:21 PM