Brake booster noise, please help

It doesn't sound very good to me. Normally the pump runs to pressurize brake fluid and stores it in an accumalator. The brake computer uses that stored fluid to actually make the brakes apply by directing it to the calipers at each wheel. It is normal for that high pressure fluid to seep some internally just sitting there so the pump has to run now and then to build pressure up- more so when you use the brakes.
The thing is on my Gen2 I can only hear the pump run when I first ready up the car. Otherwise when I am driving I don't notice it at all. I don't know if your pump cycle time is correct or if it indicates a problem. The fact that the operation of your braking system has changed would make me suggest that you at least have it checked out by someone who is familiar with the Prius.
Often, a noisy pump IS an indicator of a failing actuator, but some people have had that for months (or years) .

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