The noise you hear is coming from the brake actuator (the hydraulic power source portion of the brake actuator to be exact)
Consisting of a pump, pump motor, accumulator, relief valve, and accumulator pressure sensor. The hydraulic power source portion generates and stores hdraulic pressure, which the skid contorl ECU uses for controlling braking. The rattling noise you hear is caused by a need for more hydraulic pressure for the accumulator. In stop and go traffic is going to be a lot more noticible, also if you park your car in the garage and wait about three to five minutes you will hear it as well. (you will also hear a vacuum noise coming from the car after a two minutes). Frankly, just be ready to hear a lot of interesting noises from the Prius. Brakes, Coolant Storage Tank, A/C Compressor, EPS, Reverse Gear (BEEP, BEEP, BEEP).