Thank you Thakeray, for the in-depth post, and the super-useful picture!
I wasn't aware of of the parking pawl, and that may very well have been what I heard. The only comment I would make, which I didn't say, was that the car was on flat ground at the time, but I guess there may still have been some tension on the pawl at the time it was moved to Drive.
I have since been driving the car today, and haven't heard the noise again. I have been getting used to the normal 'operational' sounds of an automatic transmission.
Separately, I did notice that the first couple of minutes of driving the car today, the engine sounded a little 'rough'. Then, after a few minutes of driving, it had returned to being quiet and refined sounding, and was then fine for the rest of the day. Will see if similar tomorrow.
Reverse parking on a fairly steep hill was also an experience! Seemed like there wasn't much torque available at first, and then with only gentle pressure on the throttle, the car speeded backwards! Luckily I had a lot of space behind me to brake in time, but it was a little disconcerting.
Still figuring out what is normal in this car...