OEM pads (non metalic) are generally quieter than metalics and other compounds. The Mercedes brake systems are somewhat sensitive to non spec materials... probably best to go with OEM or Balo rotors and Textar pads. Sometimes the squeal will fade away and some squeal problems just can't be worked out any way but new spec parts. Till then just live with it... takes a little luster off an SL but at least the bumper won't get crumppeled.
