Probably warped rotors as others have said. If you have a lot of pad and rotor left you could try to bed them again otherwise get high quality replacement parts. Make sure your wheels are torqued correctly too. I had the same issue, usually during light braking coming off the highway, but seemed intermittent so I don't know if it was just rust or dust on the surface of the rotors and a highway run was enough to deal with it. Dealership wanted a ridiculous amount to use their specialized "tool" to resurface the rotors with the car on a hoist (special tool = emery cloth?).It is a big heavy car but it has the super track pack heavy duty brakes but I take it there are better options than replacing them with factory hardware.