Here yer go
Since their adoption by most manufacturers, primarily for their diesel models, a dual-mass flywheel (DMF) differs from the more conventional ‘solid’ type, by having a flexible damper built within it. Intended to absorb stresses from the engine and transmission, they can fail prematurely, causing a metallic ‘clanking’ sound and/or vibration to be felt at certain engine speeds. It is recommended by many technicians that dual mass flywheels are replaced at every clutch change. Still, excessive clutch slip can put undue stress on the dual-mass flywheel and reduce its operating life.
Basically its the springs inside it that wear.