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Question about a high frequency vibration sound

Discussion in 'Technical Q&A' started by italiafan, Jan 23, 2020.

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  1. italiafan

    italiafan F1 World Champ
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    Jul 19, 2006
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    Stickbones Swagglesmith
    #1 italiafan, Jan 23, 2020
    I recently purchased an 812SF with 100 miles on it at delivery.

    At ~4500-5000 rpm there is a very brief high-frequency sound that sounds like a mosquito in your right ear coming from behind and above the driver. Near the upper edge of the hatchback. It seems to me like there must be something vibrating at exactly that rpm that then creates an harmonic.
    It lasts for half second on the way up, and down the revving in that range. It happens in gears 1-6 (I haven't used 7th at that rev range yet).
    It doesn't happen 100% of the time, but > 50%.
    I have not yet tested to see if it happens stationary in neutral.
    Not speed dependent since 4500 rpm happens at different speeds depending upon the gear.

    Question:
    What is rotation-dependent when the engine is running, apart from the engine?
    If trans-axle or DCT then I would think speed dependent not rpm dependent.
    I was postulating that the exhaust system will vibrate and maybe that creates an harmonic.

    I just wanted to pick your collective wisdom before going to dealer.
     
  2. Ferrari Tech

    Ferrari Tech Formula 3

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    Wade Williams
    #2 Ferrari Tech, Jan 24, 2020
    Rotating engines go through harmonic cycles. When I balanced engine assemblies, they would spin and only shake as they went through a cycle. If I remember correctly, most would start at 400 ish RPM. As the balance was performed and the imbalance was reduced, I could spin the assembly through the shake. As it slowed it would go through the shake again. As a running engine increases in speed it goes through these cycles at different speeds. 308 engines have one at 3200 and can have all kind of sounds and rattles at that engine speed. It is likely that there is a harmonics cycle at 4500 and your car has something reacting to it. Hopefully they can find what it is and tighten it or dampen it.
     
  3. italiafan

    italiafan F1 World Champ
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    Stickbones Swagglesmith
    #3 italiafan, Jan 24, 2020
    Thank you.
     

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