Knocking Noise Seems To Come From Rear Axle

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by cooltraders, Nov 13, 2016.

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  1. cooltraders

    cooltraders Junior Member

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    To all you Prius techies. I have an "05" Prius with 174,000 miles. It runs great although at the 160,000 mark we started noticing a weird barking or farting noise when stepping on the brakes. It never caused any issues with the exception of some dashboard warnings when we either washed the car, drove through a bad rainstorm, or went through a car wash.

    Now we are hearing this weird knocking noise coming from what appears to be the rear axle area. Please note: If you are inside the car the noise also sounds like it is in the front passenger side of the auto. It seems to vibrate through the cars framework. I'm including a video with audio.



    Additionally, this sound is only when you have the foot off the brake. If you put pressure on the brake the sound goes away. Also, this sound will even continue with the car in the "off" position and with the drivers side door open. It seems like some electricity starts up the "whirring" sound of the inverter and therefore starts up this tapping/knocking noise.

    Can someone please tell me what I'm dealing with?
     
    #1 cooltraders, Nov 13, 2016
  2. Mendel Leisk

    Mendel Leisk Senior Member

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    When trying to play I get this:

    upload_2016-11-13_15-45-17.png
     
    #2 Mendel Leisk, Nov 13, 2016
  3. cooltraders

    cooltraders Junior Member

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    Here is the video.
     
    #3 cooltraders, Nov 14, 2016
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  4. andrewclaus

    andrewclaus Active Member

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    If you open the hood and listen at the passenger side firewall area while this noise is happening, can you hear anything?

    Have you ever had the brake fluid flushed and changed? What does the fluid look like? Is it honey-colored or darker?
     
    #4 andrewclaus, Nov 14, 2016
  5. cooltraders

    cooltraders Junior Member

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    Answer: I just went outside and opened the drivers side front door. I did not turn the car on at all. A "whirring" sound from what seemed like behind the hybrid inverter started up and the tapping started as well. When that whirring sound stops the tapping stops. I listened by the passenger firewall and could hear the sound but it seems like it is coming from the rear of the car near the hybrid battery. When I lightly press on the brake pedal the whirring continues intermittently but the tapping sound stops.

    The brake fluid has a honey color and has never been flushed. I'm including a full walk around video. The car was never turned on for this video.
     
    #5 cooltraders, Nov 15, 2016
  6. Mendel Leisk

    Mendel Leisk Senior Member

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    With all these videos (excellent by the way!) someone has got to have ideas. Maybe @Patrick Wong will comment?

    Does second gen have electrical pump for rear brakes, sim to 3rd? Failing? Definite clue when you step on brake pedal, and knock stops.
     
    #6 Mendel Leisk, Nov 15, 2016
  7. andrewclaus

    andrewclaus Active Member

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    It sounds like the ABS accumulator. There is a recent warranty extension on that. You may want to get it to a dealer and have them assess it for warranty, maybe you'll get lucky.

    I wouldn't drive it any more than is absolutely necessary. Personally I wouldn't get in any vehicle with a possible brake problem.

    The accumulator has an electric pump that's trying and failing to maintain pressure in the system. That's the whirring sound under the hood, not the inverter. Normally that happens every couple of times you apply the brakes. The electric pump replaces the vacuum reservoir in non-hybrid cars. When the pump runs, it adds pressure to the brakes and that's the creaking noise in the rear axle.

    The honey colored brake fluid denotes a lot of contamination, which may lead to expensive and dangerous ABS failure.
     
    #7 andrewclaus, Nov 15, 2016
    Last edited: Nov 15, 2016
  8. GreenApp

    GreenApp Junior Member

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    This is an old thread, but for anyone experiencing this, your first steps are:

    1. Check your rear brake shoe wear.
    2. Check your parking brake is working/locking
    3. Complete brake fluid flush
    4. ABS unit replacement (Not easy, not fun nor cheap)

    Solutions: (in order)

    1. Go to a brake mechanic or DIY
    2. If you never use your parking brake and it doesn't lock, pump the parking brake alternately with the regular brake until the parking brake "locks". It could take 50-200 times. If it flops to the floor and doesn't come up, your parking brake line is broken.
    3. Self explanatory. Have it done professionally, or do it yourself. It's pretty easy. You need to remove ALL of the old fluid/air.
    4. As explained above, but this warranty extension is now over. I believe much of the problems people have with their ABS units is OLD fluid and AIR in the lines.
     
    #8 GreenApp, Feb 1, 2024
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