Help me understand what I'm hearing

I know this is probably a normal sound of the prius hybrid system but maybe someone could explain what it is and when to expect it.

Yesterday was the first day I actually drove my new prius in center city conditions. Before that it was pretty much suburban usage. I also didn't have any radio or climate control on so I was able to hear everything the car was doing.

When creeping along in traffic the car would repeatedly emit a sound that I can best describe as a very tame whirling/ratchet sound starting a a higher RPM /pitch and slowing to lower RPM/pitch. When it occurs, it lasts for about 1.5 seconds at most. It wasn't loud and with the radio on or road noise I don't think I'd have heard it before. Since I was on pretty smooth streets and running a low speeds I did hear it and have to assume it was always doing it to some lesser degree but now in stop and go situations it was occuring more frequently and in the quiet... will you get the picture.

Watching the power flow screen it seemed to correspond with the times when I applied the brakes while the traction motor was delivering power to the wheels. The state switched to recharging the battery when I applied the brakes obviously.

I am assuming that this was regenerative noises that are normal but since braking didn't always cause this sound I was just wondering if in fact it is normal, what exactly is the system doing, and what algorithm or situation elicits it. When the energy flow screen already had the battery recharging and I applied the brakes the sound didn't occur. I think it was just during the state changes from delivering power from traction motor to delivering recharge to battery but I am just guessing.

Thanks in advance for the explanations and hopefully the reassurances.

 

Thank you to all who have taken the time to reply to my post. I'm glad I found this site. I've learned so much from other owners here and it has helped me to better understand my new vehicle.