Brake system pressurizing while driving?

It isn't driving that makes the whirring, it's braking. Each time you press the brake pedal it uses some pressure from the brake system. Eventually this pressure is restored by the brake accumulator pump, which is that whirring you describe.

The pump running every third time or so is normal. It should not run continuously.

Tom

 

If it runs continuously it means your brake system has a pressure leak.

Running every time you brake could be nothing more than your braking habits.

Tom

 

Your car isn't old enough yet, but it -can- run -every single time- you press the brakes. This indicates the pads are worn and the system has to move them farther, taking more volume (it's not pressure, it's volume that triggers the use of the system pressure - in other words, if your system only had to move 1 cc of fluid instead of 10 cc the pressure would drop much less and the pump would run less).

Oh, and you're much too sensitive to the sounds the Prius makes. I recommend loud rock music with earbuds to remedy this.

 

Thanks Tom, it doesn't run every time i brake that i recall

 

Disc brakes self adjust with an ingeniously simple system. They don't move further when they are worn.
This will save me a bunch of typing
Disc brake calipers

 

-When they work as they should-. Good luck with that.

 

In 46years of driving cars with disc brakes I have never had them not work. Square o-rings are pretty reliable. If they don't work, your pedal will go right to the floor when the pads wear a bit.

Edit: A little clarification. I was referring to the self adjusting feature of hydraulic disc brakes, not the entire disc brake system.