Due to the size and material used on the brakes of these vehicles, the brakes may start to squeak around the 50 percent wear point. The brake rotor surfaces become uneven, causing a lip to form at the outer edge of the rotor. Because of this, the rotors are usually replaced when the pads are worn (pad life varies depending on driving style and terrain).Google+
Car Problem Reports
Mercedes-Benz SL550 Replace Brake Rotors When Pads are Worn and Squeaking
Mercedes-Benz SL550 Problem
Average Mileage: 25,100 mi (20,200 mi - 30,000 mi)
Visitor, 2007 Mercedes-Benz SL550, 30,000 mi
My mechanic told me that a solution is to grind the rotors to remove the lip on the pad covers. the lip on the covers of the pad is what causes the squealing when it touches against the rotors.
Visitor, 2007 Mercedes-Benz SL550, 20,200 mi
Even after pad replacement brakes continued to squeal. Had to have the rotors replaced to stop the noise. Seemed very early on for such a major expense.