It would be odd for one easy ride to warp a rotor. Double check your wheel and make sure it is seated completely in the drop out. Make sure that the rotor itself was mounted on a flat, clean surface. Re-torque the mounting bolts and re-adjust your caliper. If you still get a cyclic, intermittent rub, identify the exact place that it is happening and with minimal pressure you can usually straighten the rotor. It's important to remember that the brake is not really creating any significant drag, but if you are like me the sound will drive you nuts. Good luck.