noise when braking, even when engine is off

[TonyD50]

Hello. I have a 2008 Prius, bought in April. I've recently noticed a relatively loud noise coming from under the hood, directly in front of the driver. It occurs when braking at low speeds, and sometimes on acceleration from a complete stop. For the first few minutes after turning off the engine, you can hear the noise when you step on the brake. It sounds like a very rapid, machine-gun fast clicking. Sorry for the poor description, but it's the best I can do. A Toyota mechanic said that it's the ABS pump, and that it always made the noise, and that I just didn't notice it before. I'm concerned that he might have blown it off. Has anyone had this problem? Thank you!

[realale]

We've noticed this too. My searches on the forum indicated it was something to do with the brake system as well.

Mine seems to be more prevalent when I'm carrying more passengers or cargo, but it definitely happens from time to time on both braking and acceleration when it's just me in the car as well. I'll be interested to see what the more experienced say. It definitely makes me nervous.

Great description of the noise, by the way! I always think it sounds like a Vibraslap (most like the LP 209 soundclip, but not as metallic, on this site: Latin Percussion: LP Vibra-SlaP II ). Ours lasts about 1.5 seconds each time.

[Patrick Wong]

The noise likely is from the brake pressure accumulator pump. If you repeatedly depress the brake pedal while the car is READY, you should hear the pump running to rebuild pressure in the system. If that is the same sound that you expressed a concern about, then no worries.

[qbee42]

This is the brake accumulator pump. The power brakes on the Prius are driven by an electric pump, since the engine does not run continuously as it does on a normal car. The accumulator pump kicks on about every third time you use the brakes. It builds pressure in a little tank, which is then used to power the hydraulic system. The accumulator pump makes a buzzing or ratcheting sound.

The other noise you hear after powering off is the coolant transfer pump. The transfer pump saves hot engine coolant in a thermos bottle, which is then used later to preheat the engine. The coolant transfer pump makes a whirring sound.

Tom

[TonyD50]

Thank you for the input. I have an appointment with a dealer today. I'll let you know what they say.

[TonyD50]

Well, the mechanic who worked on my car doesn't know quite what's wrong with it. He thinks the noise is much too loud. They kept the car and put me in a loaner. I'm glad that this dealer isn't blowing it off, as did the dealer who sold me the car.

[TonyD50]

Well, I finally got my car back. It took 3 days. They said that the noise was from the "electronic brake booster." They contacted the "tech line" and were told to reset the linear calibration. They did that. Nothing has changed. The noise is the same as before they made the adjustment. I've been told that it's not a safety or drivability problem, so I'm going to ignore it.

[qbee42]

It's the brake accumulator pump. I have this comment on a hot key, so I can press it once to answer this question. It's one of the most common.

Tom

[Large]

Originally Posted by qbee42

It's the brake accumulator pump. I have this comment on a hot key, so I can press it once to answer this question. It's one of the most common.

Tom

Thanks, Tom! for the clear reply to the question.
I have found this in various forums, so I will ignore this noise, unless it gets worse or I feel it affects performance. It's just unusual to have this noise when you hit the breaks (as you say, approx every third time or so and sometimes when accelerating out of a stop).
But I guess it comes with driving around with a lot of technology under the hood and the seats and the trunk and everywhere.