So i know that when you open the driver side door after the car has been sitting awhile (2010 Prius), it makes a whirring noise and that's the brake system pressurizing before you start the car. But this whirring continues as i drive the car at lower speeds within 5-10 minutes of starting on my way to work. The first mile is pretty much down hill, only need the ICE for about 1/5 of that distance, and use regen braking all the way. When i come to a stop sign - there are 3 in that first mile - the whirring sound kicks in. I notice it most when i'm stopped, then slowly creep forward as the car ahead of me goes through the stop sign and it's my turn to stop, i'll hit the brakes for that last bit and hear the whirring. Is this normal after the has been started and driven for a short bit?