Problem solved - peace and quiet restored! Many thanks for all replies, particularly PMR and pobrown who put me on the right track. The culprit was indeed the Dash Cam that the dealer persuaded me to have fitted. The answer is in the Dash Cam settings, accessed through your phone, where the start up and shut down racket can be muted. Thanks again. This owners' forum is a terrifice resource to solve problems.As an aid to others who may experience the same problem, I've attached here a video of the noise before it was suppressed.
20191104_084810_272234244830396_(1).mp4