Every time I go to apply to brakes, I hear what sounds like birds chirping from the right side of my G for the initial brake application. I think I've narrowed it down to the back passenger side wheel, but very well could be the front. I don't think it's the brake pads because I replaced all of them on every wheel about a month or two ago. My rotors do look a bit beat up on all my wheels though, so it might be that. Anyone have any idea what it could be?