2005 PRIUS Whirring Sound When I Take My Foot Off The Gas Pedal

To all you Prius techies out there, I have a "05" Prius problem that I'm stumped on. I have 165k miles. I am the original owner and have maintained this car really well and it really runs incredible. I just started to hear a strange new whirring/winding sound coming from the front when driving. When accelerating it goes quiet. When I let off on the gas it starts whirring. Between 40-45 MPH if I let off on the gas it will whir but then when I drop below 40MPH it does it intermittently. It almost sounds like it is attempting to slow the car down . I've looked over the forum and other sites and hear about other sounds like this (ex: A/C motor), but nothing that matches my symptoms. I can say this, it in no way affects the performance of the car, just very annoying. If it is serious I would like to jump on it right away and try for 200K . Thanks for any help you can shed on this problem.

 

Thank you. I read somewhere that it is the Regen battery charging or something like that. Is that what you are referring to? Great car, just sometimes very complicated. Is there a way to clear that up? Other than that the car runs perfect. Is this a sign the hybrid battery is weak?

Thank you for the help and quick response.

Scott