Tony, I've had a similar issue and posted about this a while back. I took my car into Porsche and drove around for awhile, and fortunately it happened while the tech was in the car (West London Porsche). Some adjustment was made and it is much improved, but certainly still present.
Having done some experimentation, I believe it is related to either the wireless connection between the key and the car and/or the PVTS...which I suspected from the beginning as it sounds like feedback to me. If I open the car door with the key, and then put both the key and the PVTS token in a Faraday pouch (which I use as an additional layer of security against a relay attack) then the sound is quite loud. Perversely I am glad to hear that I am not the only one, I was told that the Porsche service staff had heard of this previously although they did not give me feedback on what they believed the issue was (I picked the car up after the staff had left).
May or may not be the same issue...