john-e89 said:Can't help wondering if it's window related Andy, if the window is not quite sealed against the rubber but as the speed rises the outside air pressure pushes the glass in sealing it. Just a thought.
I'd experiment with this - dropping the window a tiny fraction to see if the noise is affected. Would fit with only hearing it when sat upright. If so then it's rubber seal conditioning time or even replacement?
Hope you get to the bottom of this - I can imagine it is the type of thing which will drive you nuts! Turn the music up?!