Pumping noise when pressing on brakes

Hi all,

I purchased a used 2nd gen 2006 Prius with my partner with around 200k miles.

When I press hard on the brakes (either when parked or in motion), I hear a pumping noise every second or so. I've gone through the forums and I've seen it described as a "barking" noise. Otherwise the brakes sound normal when it's pressed slightly.

Is a part like the brake accumulator going bad or should this be something to be concerned about? There's no check engine or warning lights on the dash. We've been spending time at Lake Tahoe and I also read that altitude might cause an issue as well.