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Thread: Loud buzz in engine compartment

  1. #1
    Join Date
    Jun 2003
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    Default Loud buzz in engine compartment

    Lately, after turning the engine off there is a loud buzz coming from the engine compartment. It lasts for 15-20 minutes or so and seems to come from the area under, or near, the air filter box. It is fairly loud and difficult to isolate so I am not completely sure about the location.

    Any ideas about what this is?
    Wait Griswold
    2003 XC70
    South Lake Tahoe, CA

  2. #2
    Join Date
    Mar 2005
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    Default

    I know of only 3 electrically controlled valves around air box that can probably buzz. TCV (attached on the air box), ABS is under it but towards the firewall, and EVAP purge valve (buried near bottom of airbox for an 03 I think, 05+ is under intake manifold near #5 cylinder)

    Use a tube from a paper towel roll or long screw driver to see if you can isolate it.
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  3. #3
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    Default

    Here's a few techniques to localize an abnormal vibration sound.
    e-stethoscope: wearing earbuds, probe around engine using cell phone's mic (e.g., voice memo app)
    old school stethoscope: use 1970s era airline headset; touch components suspected of vibration
    vibra-stick: use handheld aluminum chopstick poking around to feel for source

  4. #4
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    Thanks. I will take a closer look.
    Wait Griswold
    2003 XC70
    South Lake Tahoe, CA

  5. #5
    Join Date
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    Landenberg, PA
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    Default

    I had the same symptoms you have. It ended up being the Brake Booster Switch - Volvo Part # 31265825. It's a switch next to the airbox. Disconnect the wire harness, if the buzzing stops, that's it. Simple to replace.


    2005 XC70 - 50/35 Tint, Yakima Rack

  6. #6

    Default

    Its not a fan sound is it?
    I believe there is a computer fan of some sort that runs post shut down - mine does it from time to time but genuinely sounds like a fan.

  7. #7
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    I never heard of that before, only the engine cooling fan.
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  8. #8
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    Best $20 you'll ever spend:

    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

    It has both a sound tube, and a metal probe.

    Use the metal probe to check components. The noise will conduct very clearly through the probe. You'll know what's making the noise when you find it.
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  9. #9
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    Quote Originally Posted by perkinscl View Post
    I had the same symptoms you have. It ended up being the Brake Booster Switch - Volvo Part # 31265825. It's a switch next to the airbox. Disconnect the wire harness, if the buzzing stops, that's it. Simple to replace.

    Yes, this. The brake booster vacuum pump (under the airbox) is quite loud. The switch is in front of the airbox and has a wire harness and vacuum hoses connected to it.

  10. #10
    Join Date
    Jun 2003
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    Thanks everyone for your replies. I was gone for on a trip and when I returned the buzz was gone.

    I suspect it will return and when it does I will trace it down, with all your good advice, and report back.
    Wait Griswold
    2003 XC70
    South Lake Tahoe, CA

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