The most common cause is excessive warping of the brake rotor. This is caused by improper tightening of the wheels or wheel hubs that are not properly cleaned when the rotors are installed after a brake job or tire rotation, or it could be a bad hub. The problem does not show up right away, it can take a few hundred miles of driving, or getting the brakes hot enough for the vibration to show up. To solve this problem, the mechanic must first determine if the rotors are warped, the hubs are dirty, or if it a bad hub. The rotors can be machined if there is enough material on the rotor to machine it properly without going below minimum thickness required for a safe rotor, or the rotor must be replaced. If the hub is the problem it must be cleaned properly, the rotor reinstalled, and the smoothness and straightness (run out) rechecked.