I have a two and half months old Toyota RAV4 hybrid. I have noticed that occasionally when pressing the brake pedal when being stopped, I can hear sound that seems to come from the ABS. It is definitely similar to the sound you can hear when braking hard on a slippery surface. The sound lasts for about half a second, and if you continue to press the brake pedal, the sound does not occur again.
I haven't found any logic to the sound. It certainly isn't caused when you press the brake hard: I have many times pressed the brake hard when being stopped and the sound does not occur, and many times pressed the brake very lightly when being stopped, and I can hear the sound. Most often, the sound occurs after starting the car, but occasionally I have heard the sound while being stopped at a stoplight.
My own theory is that this is some kind of self-test for the ABS that the computer occasionally performs to see everything is working.
Apparently, RAV4 hybrid is not the only vehicle with the sound. 2008 Ford Escape hybrid has also the sound according to a comment to my answer in 2008 Ford Escape hybrid brake and computer problem.
However, another comment to my answer has a different theory. According to it, "it's the actuators in the bypass modulator valve activating to allow pressure through. It should only happen if you press hard on the pedal when stopped". This certainly isn't in line with my experience: I have heard the sound many times when pressing lightly on the brake pedal, yet not heard the sound many times when pressing hard on the brake pedal.
So, my question: what is the sound? Is it a self-test as I suspected, or is it something else?