I have had this problem from before the warrenty was over until now 89K mile on my 2000 E320. MB checked -recheked and tested replaced Mounts and balancers and finally siad "it is within acceptable tolerence"
I changed 2 set of tires, one set of Rims, many dynamic road force balance to no avail, still hums at about 62M/h speed.

Now I am about to change everything on drive shaft have it balance and see what happens.
I think this is an inherent bad engineering desing by MB and they still do not know or perhaps care.

After all been done to elliminate the vibration, the only thing left is the drivshaft.

Ve