Hi Jim, sorry this is not going to be of any help but our car has had the same noise,only been doing it for about a month and seems to be
when the weather is fine,and the car is dry.
We had ours checked over by a very reputable garage not main dealer,typically it didnt make the noise the whole time,but it was raining
on the way there. All the usual checks were done and nothing was found ,it looks like i will be booking it in for a more detailed check.
Ours is a 56 plate with the same mileage, so if you find out what it is i would love to know Thanks chris