Vacuum pump running constantly and rattling?

I have another "fun" Prius behavior question for the gurus. I had the doors open while unloading the car this morning and noticed that the brake booster pump just kept running and running and running, plus I can hear a strange, hollow rattling sound somewhere under the car that coincides with the pump. When I say it runs "constantly," I really mean that it runs for 10 seconds (with the rattle), stops for about 5 seconds (silence), runs and rattles for ten seconds again, stops for another five, and just keeps on going for as long as I care to sit there with the door open.

I understand that the pump runs the first time to prime the brake system so that it will be ready when I hit Power. I'm also well accustomed to the normal buzzing and clicking sounds thanks to nearly 6 years with the car. But the extended running and the rattle have me confused, even a bit concerned. I have also heard the "bark" a few times, and the rattle sounds like something different. It isn't really metallic, but a well-damped low-pitched thunking like hard plastic against a heavy piece of the frame (or something), maybe at 10 or 20Hz. Anyone run into this?

 

Now with video...hopefully this will work. You will have to turn your volume up a bit to hear the rattle behind the normal buzz.

 

OK, it's worse. I started off on a trip to see family, and noticed that the vacuum pump now runs almost constantly while driving. 5 seconds on, 5 seconds off, 5 seconds on. There must be a vacuum leak somewhere, hopefully something simple like a critter bite. We aren't going anywhere in that car. Suffice it to say that this is not my happiest Prius moment.

 

An intermittent leak, apparently. Now that I have the thing up on jack stands to look for the problem, the original symptom is gone. I wonder if this could be some kind of instrumentation & control glitch? Guess I'll keep poking around for a while.

 

No, I'm pretty sure this is a pneumatic pump. IIRC, the same vacuum pump feeds the brake booster and operates the evaporative emissions system checks. A hydraulic pump wouldn't be appropriate for either application. The main reason for this pump is to replace engine manifold vacuum since the engine basically bounces between off and 80% throttle with little in between.

 

OK, guess I understand now. There must be some volume of gas in the accumulator to store energy, right? (You need something compressible, and brake fluid isn't.) Then if it were a leak in the accumulator itself that gas volume would be gone (in which case the pump would dead head, which it isn't) or there would be brake fluid everywhere (which there isn't). Similarly, any external fluid leak should be evident at this point considering the severity of the initial symptom. That leaves me with some kind of internal leak, probably inside the actuator assembly. And the fix for that is guaranteed to be to replace the actuator a$$$$embly. So that sucks.

On the other hand, the car isn't doing anything abnormal right now. The issue I showed in the video is gone. I also went on a a test drive around town, and the frequency with which the pump runs appears to be roughly normal.

I'm simultaneously optimistic and troubled. I expected to go farther than 5.5 years and 88k before having to worry about something like this. Yet it could have been some flukey one-off thing like a partially closed valve that subsequently re-seated itself.

 

I wound up doing all the driving that I had planned for the day...loads of highway miles...without a hint of trouble. Either I lucked out or the car is screwing with me before delivering the final blow. I'll keep you posted if anything else happens. Thanks for the help troubleshooting so far. At least now I have a clue how the system works.