It was by accident. I was on the driveway and the engine was idling. The dog was on the front seat and when I went to chuck her in the back my right foot accidentally pressed fully down on the accelerator pedal. The vibration / drone was really apparent at full throttle.
I was always under the impression that it was from the drive train..
After that it was a process of elimination and the first thing I did was turn the climate control off. Hey presto, no vibration..