Clicking Noise at 12-13mph

Not sure if this is a Prius thing or just a Prime thing (as the Prime is my first Prius), but I've noticed each time I start the Prime for the day, once I hit 12-13mph, a slight "clicking" sound occurs (almost like a soft version of doors locking) as soon as I hit that speed. The usual routine is backing up out of the garage, then putting it in D... and when I hit 12/13mph, I will hear the sound. Funny, I hadn't noticed it until recently (maybe because of noise or distractions, needing to get going) but I've noticed it lately, and sure enough, it's there at the start of using the Prime each day, as soon as I hit 12/13mph. A couple times I even tried going really slow down my residential street for a while to see if it was an "elapsed time" thing or a "speed" thing that triggers it, but it's definitely speed -- for it's only when I first hit 12/13mph when it happens.

I have no idea what the sound is. It's different than the "shhhhhhhh" brake sound one hears when opening the driver's door before you're ready to start.... it's instead a soft, short clicking sound, like a muted version of doors locking. What is it?

 

Thanks. So what are the air flow shutters then, and why would they need to change position at 12/13mph?

EDIT:
Just found this:
From Toyota Canada:

"When the vehicle is off, the grill shutters automatically open."
and
"When the power switch is on, and the engine coolant temp is below 40C, the grill shutters are closed."

So it sounds like they open when you turn off the car, and once you turn on the car (in decent weather) and hit 13mph it's the sound of the shutters closing (until they're needed to open again).

Interesting that this came from Toyota Canada (and I didn't see any such videos from Toyota USA)

 

Sorry to bring this thread back, but... to echo what benagi said, there seems to be two answers as to what the 12-13mph noise is. So... which one is it?