I have a 2009 Merceded S Class. A vibration or dull rumble when I hit 70-75 mph. Before that no vibration or rumble. Front end aligned, new tires, wheels balanced etc. Could it be motor mounts or transmission mounts?
Unfortunately it’s a common problem. At one time there were rumors of a manufacturing defect. https://mbworld.org/forums/s-class-w221/459009-class-action-vibration-problems.html