Hello! I signed up for this issue as well. Just bought a 2016 44k miles energi. Love the car but noticed the acceleration noise that OP describes. Thought it was transmission revving higher than normal but what @murphy states sounds about right. So @murphy I should not be worried about it? I thought to take it back to dealer to have them check since it's still under warranty (blue certified). Again, the problem does not happen all the time.
Glad to join this community.