Our 2010 Prius has always made this sound but it seems to be more frequent lately. It will make this sound when I first get in the car but it's doing it about every 18 seconds driving around town now. It's a whirring sound, starts out higher pitch and then winds down. Anyone know what it is and if I should be concerned? Thanks
It's the pump that repressurizes the brake fluid when the pressure drops as you use the brakes (or, in this case, when the pressure keeps dropping on its own because of internal leaks developing, usually inside the brake actuator). Internal leaks don't result in any visible brake fluid outside the system; it just leaks back to the reservoir, and the pump has to pump it up again. The pitch drops as it pumps because it is working harder as it builds toward the target pressure.
