Hey I coded the i3 so I could run the REX at 70%. While driving in city traffic I couldn't hear it running. But from what I have read, it is quite audible but not the case with my 2020 i3S REX.
Once I reached my destination, I left the i3 on and walked to the back and I still could barely hear it. I had to place my head underneath and place my hand at the exhaust port. This confirmed it is working away. It then ran for a short period and then shut down. So I don't know what BMW did but maybe they muffled it more.
The only clue I had when it started was a pungent smell. I have experience this smell of a brand new motorcycle engine so other than that, this works wonderfully!