vibration in car on e280cdi 2006
 

Just bought a 2006 E280cdi. It seems to have intermittent cabin vibration at speed. In 4 weeks it has happened twice. After the first episode i took it back to MBD andf they said they couldn't see anything wrong, but balanced the wheels and adjusted the tire presuures. Still happening...
Have seen other threads with references to drive shaft aligmentment, lower ball joints, rubber joints on mounts.

Going back to garage but any suggestions would be appreciated.