The rubber bond in the harmonic balancer can decay, which causes the balancer to move and work its way toward the timing cover. If the balancer comes into contact with the timing cover while the engine is running, it will fracture and damage the cover and potentially other surrounding parts. The harmonic balancer should be inspected at every service.Google+
Car Problem Reports
Mercedes-Benz CL55 AMG Harmonic Balancer May Fail and Damage Timing Cover
Mercedes-Benz CL55 AMG Problem
Visitor, 2004 Mercedes-Benz CL55 AMG
it is separating at the rubber. the belt is now riding on the edge of the pulley. i will replace with a new unit.