On our 2008 Prius, while the brake assist pump is running I get a steady thump-thump-thump-thump noise, as shown in the video below. If you put your foot on the brake the noise stops. The brakes operate fine and there are no warning lights on. Does anyone have any idea what this might be? It's coming from the passenger side of the car, and oddly it's loudest under the car near the back seat. 86,000 miles on the car.
wow, that's a lot of noise. not sure if it's the same as the honking/quacking threads that turn out to be the brake actuator. have you checked them? i think they say if you can live with the noise, the brakes work fine.