the aux vacuum pump allows the doors to be locked and unlocked a few times without the engine running. It recharges itself electrically. If you have a bad leak somewhere in the centreal locking it will start running even with the engine running. Its unexpected and sounds horrible. In that case somebody might just remove it. If it were removed i imagine the system might consume the vacuum present in the booster to function for limited number of times.

The vacuum lines under the passenger side rear carpet head to the booster pump you are missing. I would look under there for plugged or missing lines.