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Weird sound before start up?

Discussion in 'Gen 4 Prius Main Forum' started by Chris Wolfgram, Nov 19, 2023.

  1. Chris Wolfgram

    Chris Wolfgram Active Member

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    So I'm sitting in the airport waiting lot, it's a little cold, heater on, and I noticed each time before the car fires up (automatically) I hear a little gear sound? It's not super loud, and only last about 1-2 seconds until the motor starts, then goes away. But interestingly, right before the gas motor shuts off, it also does the little gear sound ? Granted, that is not very noticeable because with the has motor running, it's harder to hear.
    Anyway, if a person didn't know this car, they might not even notice it. But I've slept in this car enough, I'm sure I would have noticed it at some point, and I really haven't ?
    Any of you ever heard this, or know what it is ???
     
    #1 Chris Wolfgram, Nov 19, 2023
    Last edited: Nov 19, 2023
  2. ChapmanF

    ChapmanF Senior Member

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    My Gen 3 applies the brakes just before starting the engine (if the car isn't moving), to restrain the rocking of the body from the starting torque. Does the same thing just before stopping the engine (again, if the car isn't moving). Sound is the clicking of valves in the brake actuator and a faint sound of whooshing brake fluid if you listen closely enough.

    If the brake accumulator pressure is near the trip point for running the pump, the brake application will also trigger the pump to run for a second or two to pump the pressure back up. Sounds like a little rattlesnake or joy buzzer.

    I don't know for sure about your Gen 4 but I would guess it is exactly that.
     
    #2 ChapmanF, Nov 20, 2023
    Last edited: Nov 20, 2023
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  3. Chris Wolfgram

    Chris Wolfgram Active Member

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    Thank you. Sounds likely. I just never really noticed before. These cars (and maybe all modern Hybrids..... My GF has a Ford Maverick truck) make ALL kinds of weird little sounds, even 15 or 30 minutes after being shut off ! Its pretty funny sometimes, its like the car has a mind of its own :) lol
     
    #3 Chris Wolfgram, Nov 20, 2023
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  4. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    I don't know about a mind, but they do have dozens of computers.

    Lots of cars are set up to test the integrity of the fuel system several hours after engine shut-down. Lots of people got worried when they noticed a click or buzz from under their car four or five hours after they parked, but it's all normal. I recall that design became widespread around 2005, and most cars (including the prius) still have some version of that system.
     
    #4 Leadfoot J. McCoalroller, Nov 20, 2023
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  5. ChapmanF

    ChapmanF Senior Member

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    Gen 3 does a little brake self-test 90 seconds after power off. Click squish, clickclick squish click. (+ rattlesnake if the pressure was low enough.)

    Pretty much the exact same sounds as the brake application before starting/stopping the engine.

    So if you just power the car off and sit there for 90 seconds, you can hear it, and go "oh yeah ... that sound."

    (Assuming Gen 4 does the same kinda test, with the same delay after power off.)
     
    #5 ChapmanF, Nov 20, 2023
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