I don't know if this is your noise but it seems synergistic with your description. I have what I would call a drone sound when the internal combustion engine is running (ICE) and it is regeneration mode. When I first bought the car it did not have it, but after one of the early software updates it started. It may be they changed things to be more aggressive with recharging the battery. I am used to it but if it did that when I tested the car I would think the car was noisy. I am not sure if the later models have it but it is my understanding is that it is common on the 2013s. My drone/moan has slowly changed louder. Regeneration of the HVB(high voltage battery) mode can be identified with an up arrow thingy on the empower screen. This condition probably is mentioned in one or more message threads on this forum but it not a situation that is worrisome to me.
What is of concern is the 2013 transmission problems which there are also message threads discussing that.
Edited by obob, 01 February 2018 - 02:21 PM.