Only a guess as of now, but the noise may be a result of the higher differential pressure drop as boost pressure (accumulator pressure) is released to a lower pressure at altitude (larger differential drop). These types of systems operate differently at altitude, with the compressed nitrogen gas pushing brake fluid to a lower atmospheric pressure. This pressure differential can result in resonance across the flow orfice. Also, when going from lower altitudes to higher altitudes, the net oxygen content absorbed into the brake fluid is higher, and gets released when the fluid under pressure is released during braking. It takes several pump run cycles to fill the accumulator with the oxygen content available at the current altitude. The oxygen release has resulting in noise or other interesting effects previously. I don't have enough altitude here to prove this out, however temperatures have dropped to below zero. This often causes noise issues as well as the brake fluid viscosity changes (fluid gets very thick). I will pay attention and try to determine if the result is similar to the other systems I previously worked with.
I haven't noticed it. Probably because the car I drove just prior to my Prius was a 1996 Saturn that was just about due for new brake pads and would make a very high-pitched 'screeeeee!' every time I applied the brakes. That was a horrid sound. But I couldn't justify spending $ on getting new brake pads when I was a few weeks away (at the time the sound began) from taking delivery of my new Prius and selling the Saturn.
