EXACTLY 1100 RPM HUMM / RUMBLE Sound

Discussion in 'Engine' started by 430S, Jan 13, 2016.

  1. 430S

    430S New Member

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    S430
    Hello gents /ladies,

    Sorry to be asking for help in my 1st post :(

    I recently caught some ice and gently (very gently - 5mph max) put my S430 into a shallow ditch. Car was recovered by tow truck by front axle - had a few errors such as airmatic / ABS kick in but went away upon engine restart.

    However i now get a humm and rummble which sounds like its coming from the rear at EXACTLY 1100 revs. Less than that - no noise / more than that - no noise.

    Cant even imagine what it could be... was running whisper quiet before the incident.

    Upon start up - as they all do - it sits at 1100 revs warming up so now noisier (not super loud mind you but noticeable compared to how it used to be - but again sound seems to be coming from rear or middle not from the engine)

    Any ideas what this could be before i take it to the dealer?

    Car is super super serviced and looked after - so im pooing myself at more expense..

    Any help is much appreciated!

    James.
     
  2. LTD

    LTD MB Club Veteran

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    Yep, one of THOSE !!!
    Does the rumble happen when stationary or moving ?
     
  3. OP
    OP
    430S

    430S New Member

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    thank you for the reply! Both - when warming up, when warm and revving, and when moving ..... but it does seem louder /rummblier when moving...
     
  4. LTD

    LTD MB Club Veteran

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    Yep, one of THOSE !!!
    Was the ditch soft mud or a hard surface ?

    Given that the vehicle shows this whilst static, this rules out the drivetrain (gearbox and propshaft/diff) and wheels/bearings.

    I'd check for any loose bodywork or underbody panels - even as simple as checking the insert for the tool kit/spare wheels.

    It is a very low RPM for resonance so points to a large area that is vibrating.
     
  5. OP
    OP
    430S

    430S New Member

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    Soft / muddy / grassy... clumps of mud & grass stuck under car when it was dragged out (essentially the car bottomed out @ centre of vehicle when at an angle in the ditch)...

    Had the car washed - all totally crisp in terms of body work (not a single mark) but i'm thinking it could do with an under body wash with a pressure washer.

    I just dont get why the sound / rumble is at exactly 1100 rpm and not more or less... Do you think its something loose that's responding to a very specific vibration of the whole vehicle ?
     
  6. OP
    OP
    430S

    430S New Member

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    Apologies -what i meant to ask is: can it really be something as simple as something being loose but responding to such a specific rev? Thank you!
     
  7. E55BOF

    E55BOF MB Club Veteran

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    Could it be that the exhaust has shifted slightly and is just touching something at those revs? Any chance of getting it up on a ramp, i.e. with suspension in normal position, and standing underneath to try to identify the location of the sound? Your local tyre fitting place might be willing to let you do this.
     
    Last edited: Jan 13, 2016
  8. LTD

    LTD MB Club Veteran

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    Yep, one of THOSE !!!
    It could be - most rattles or resonances are mainly due to something vibrating at a particular energy input.

    Given it happens when your vehicle is static and all was well before the incident then I would think that something has altered.

    If you have W220 (and I had one) then there are big underbody panels that may have came loose. It might even be a slack wheel arch liner.

    Hopefully something simple yet probably the worst sort of problem to rectify.

    Methodical checking is the only answer and pressure on items will alter any resonant reaction (Q) so that will be detectable to the ear.
     
  9. OP
    OP
    430S

    430S New Member

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    I will check as best i can by eye tomorrow. What is the best way of confirming movement / where should i push it, pull it from? Is there supposed to be any play at all? Again thank you.
     
  10. OP
    OP
    430S

    430S New Member

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    I mean in terms of the exhaust.


    I will also check underbody panels + wheel / arch liners for anything obvious and update.

    Expect it's too soon to take it to the dealer as they will most likely "try" things.

    THank you!
     
  11. Erfan

    Erfan Hardcore MB Enthusiast

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    Move the exhaust up and down and side to side a bit to see if noise disappears. Do when first starting up to ensure exhaust is not hot. There's normally a bit of play.
     

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