Just bought a long wanted 300cd. Nice one with 133k and perfect maintenance by MB dealer. Driven conservatively by one owner. Everything great except car has light resonance/rythmic vibration at speed. Have replaced driveshaft center bearing, engine mounts, transmission mounts. Wheels are balanced. Local MB dealer says I need new driveshaft. My questions: how can drive shaft go bad on such a pristine vehicle? What else could cause this irritating vibration. Thanks. Nic in S.C.