I used my REX a couple of days ago, and when it is running on "low speed", I can barely hear it (if at all). When REX kicks into "high", then I can hear it, but it sounds like a scooter or small motorcycle outside or next to me, but it is still not loud or obnoxious inside the cabin. If I roll the windows down, you can of course hear more, but still not bad.
Oddly, my 2019 REX was a wee louder than my 2020 REX....but I think I had a "bad" 2019 in terms of sound insulations. In my 2019 the front doors rattled and the dash too and many times the moonroof covers. My 2020 actually feels like a BMW