Yesterday i was driving to work and my front right tire started to screech. I took my tire off and couldn't find a rock anywhere cause i live on a gravel road. The rotors were smooth so I don't think anything is in there. The bearings are still good so its not that. The rotors heat up very fast though while driving and takes about two minutes for the screech to start. It happens whether I am applying the brakes or not. Any ideas? Any help would be great

 

the brake piston is probably sticky. Some dirt probably worked its way in and is keeping it from fully retracting properly.