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.
Car Problem Reports
Mercedes-Benz ML350 Harmonic Balancer May Fail and Damage Timing Cover
Mercedes-Benz ML350 Problem
Visitor, 2003 Mercedes-Benz ML350, 47,900 mi
My harmonic balancer failed. I needed a new water pump, coolant lines and a belt. When the balancer failed my fan started to vibrate and one of the belts broke off and became wrapped around the fan.