Trust me, it is going to be one of the front rotors even if they are new. A Wagner brake products rep told me if the rotors are laid down flat at the parts stores, they can warp after a long period of time in that postion. Through the years I have encounterd brand new rotors that were warped when test driving a customer's vehicle. If you did the work yourself then simply take both front rotors back for an exchange. Also clean up your caliper pins/slides and apply caliper slide grease on them to prevent any premature wear or warpage.