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Low drone/hum noise at 1700RPM?

Discussion in 'A4/S4 forum(B5 Chassis)' started by Spacer, Sep 22, 2011.

  1. Spacer
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    Spacer Member

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    [Sep 22, 2011]
    Hi all,

    I have been getting this humming noise coming from the car when the revs hit between 1600 - 1800 RPM (almost exactly 1700). I don't get it if the car is out off gear and I rev it up as I've tried it's only when in gear and driving. It's sounds like a deep drone or hum and only lasts a few seconds whilst between those rpms but as those rpms are regular revs to be in I get it quite often. Whilst not overly loud it is loud enough to catch my attention.

    Tried all kinds of searches over the past few months but can't seem to find anyone else with a similiar problem. Anyone here have any idea what it could be? If I was to guess I've always thought it was the CAT Converter but that is only a guess.

    It's a B5 1.9 TDI SE

    Not sure I have explained it the best here so if anyone needs any further information just ask. It's hard to describe a noise.

    Thanks in advance
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  3. AR-rkon
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    AR-rkon BZZZZ.... caffeine fueled electrical engineer Regional Rep

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    [Sep 22, 2011]
    funily enough my 1.8T has the same issue, but somewhere in the 2600-3000 range... been driving me mad so would be nice to see some ideas as to what is causing it.
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  4. FactionOne
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    FactionOne Administrator Staff Member Administrator

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    [Sep 22, 2011]
    Don't wish to be the angel of death, but aren't those bands roughly when the diesel and petrol turbos respectively start getting work in earnest?

    Presumably something which is affected only when the engine's under load if it doesn't do it in neural? I guess that doesn't automatically discount the cat though, as I suppose pressure and EGT will be greater under load - and I had a company Passat (B6 1.9TDI) with a faulty cat which only showed symptoms when there was load (a cuckoo noise when backing off abruptly/dipping the clutch with load built up).

    Just thinking laterally, maybe there's a way to induce limp mode (could the N75 be disconnected?) to allow you to test with load on the engine but not the charge system in the hope of narrowing it down a bit?

    (I'm afraid I'm only an expert in terms of an ex being a has-been and a spurt being a drip under pressure)

    Rob
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  5. Spacer
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    Spacer Member

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    [Sep 26, 2011]
    Thanks for the reply Rob. I can't agree or disagree with you on what you say as I am not an expert myself heh. It is either a vibration noise or it's coming from the CAT in my opinion. Why only at those rpms specifically? When thinking about that question and if what you say is correct then it could explain why the noise isn't random but specific to a range.

    If anyone with more knowledge of the engine can think what would cause a vibration noise at specific rpms? Are there any other signs it could be a turbo issue?
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  6. aragorn
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    aragorn "Stick a V8 in it!" Staff Member Moderator VCDS Map User

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    [Sep 26, 2011]
    I recon its just a resonance. Low RPM's, coupled with transmission and exhaust noise etc will all just be combining together at that particular frequency to create a much louder noise.

    Its perhaps being accentuated by the dual mass flywheel and engine mountings all becoming a bit saggy.

    My 1.8T has a solid flywheel, and i find that applying load at very low RPM's (sub 2k) causes a lot of cabin vibration. The engine doesnt sound happy almost like a groaning sound. Once the revs are above 2k it goes away. It'll coast along just fine at 1500rpm, but as soon as you apply some throttle to try and maintain the speed say up a slight incline or pickup speed slightly it drones away. Its these types of noise the Dual Mass flywheel is designed to dampen, so it doesnt bother me, but it may indicate yours is becoming worn.

    Do you get any judder when drawing away, which might indicating the DMF is knackered?
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  7. AR-rkon
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    AR-rkon BZZZZ.... caffeine fueled electrical engineer Regional Rep

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    [Sep 27, 2011]
    It'd better not be haha it's now a single...
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