There's nothing wrong with your car. 2003 and earlier Prius models don't make this noise because they rely on the engine's vacuum to keep the brake booster charged. 2004+ Prius models use an electric pump that periodically activates to keep vacuum replenished for the braking system to prolong ev-only driving.

It should sound like an aquarium air bubbler (if you've ever heard one) buried deep in the dash on the driver's side. It comes on every 60 seconds or so in urban driving.

To test: Turn your car off. Pump the brake pedal four or five times. If the buzzing starts, then this is normal and to be expected. Make sure to let the coolant-storage pump finish its thing after the car shuts off to avoid confusing the noises.

-Drew