Thanks for the suggestion, I was led to believe that it was a vacuum leak noise and have spent hours trying to trace it. I took the battery and its tray out to follow a grey vacuum line which enters the cabin under the left corner of the windscreen / scuttle area. Inside the car it leads to a vacuum reservoir behind the glovebox (RHD car) and feeds the re circulation flap when the button is pressed. This reservoir is often wrongly assumed to be part of the central locking system. I disconnected this grey pipe from the cabin so I know this isn’t where the noise is coming from.