Buzzing sound after shutting down

I've seen a few other posts on here about various sounds, but this one hasn't been described yet...

About 30 seconds after I power off the car I hear a buzzing sound. It sounds like it's coming from under about the center of the car, but of course as I learned with my last issue it's hard to tell where a sound is coming from. And it lasts for about 1 minute 15 seconds.

If it had done this from the very beginning, I would have just assumed it was a normal Prius sound, but I don't remember hearing it at first, and only noticed it about a week ago (so I'd been driving the car for about 2-3 weeks before noticing the sound).

Just for reference, when I first open the driver's door after it's been sitting for a while, I hear a sound that's more of a vibrating pumping sound. I'm assuming that's the break system building pressure in preparation for driving. The sound I'm noticing now is different than that one.

Any thoughts or confirmations that other 2010 Prius are making this same noise?

Thanks.
-Alex