So, after 10 days, I drive round to Maplins for some bits and pieces, get back into the car, listen to the whining and buzzing for a few secs. decide that the weather has improved enough to put the hood down, leave the engine running whilst I do so, hop back in and slam the door and all the extraneous noises promptly stop.
Despite coming home on country roads with terrible surfaces, I can't replicate the problem.
I suppose it's not the end of the problem, but probably narrows it to a loose connection, or maybe it's wear and tear on the docking connector, at a quick calculation, it's been in and out a couple of thousand times (No dirty jokes, please!).
But thanks for the help gang, I may very well have to return to the subject.