I have the same noise on my 2008 +4 (and it was the same on the other car of the same age I used to own) and I am advised by a dealer I trust that it's totally normal for the reasons given above. It's disconcerting and I have been under many none Morgan's whilst they are running when up on ramps and never heard anything so loud. However as you report the noise is only there when stationary (much worse when parked in my garage as the noise reflects back of course) there are no stiff changes or gearbox issues at all. I have heard it on new cars and cars with 30K miles on the clock and it seems the same so I am sleeping easy on it.
My advice would be to keep a note of the exchanges between you and your dealer (emails and their replies) then you have proof that you raised the concern during the warranty period should things get worse later on but I think/hope you will find they will stay as they are.
