My 3.2 X300 had a nasty vibration/drone that kicked in abruptly at 60km/h. It was most obvious when slowing from speeds above 60, with no throttle - it sounded like truck engine-brakes growling, then would stop on the dot of 60 km/h.
My mechanic has cured it - a new driveshaft hanger bearing. He ran the engine up above 60 km/h up on the hoist, and could see the 'wobble' and damaged bearing. Only had to remove the whole exhaust to get to it! However problem solved, much nicer now, even zipped up over the Ton this morning without ill effect.

Now I'm just waiting for the invoice..