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Humm noise when getting off the power between 2k and 1k rpm (TDI 170)

Discussion in 'A3/S3/Sportback (8P Chassis)' started by chinnyhill10, Dec 18, 2013.

  1. chinnyhill10
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    chinnyhill10 Member

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    Hi,

    A few weeks back I posted about a hum noise coming from the steering. Well turns out it's not the steering.

    The hum noise seems to emanate from the passenger side of the engine bay whenever you let off the power suddenly (i.e dipping the clutch or revving). At road speeds its too low level to hear but when manuvering you do hear it (hence why I thought it could be the steering).

    The hum is low level and lasts for the time it takes the rev counter to drop from 2k to 1k RPM. In order to instigate the hum, you have to be above 2.5k RPM before letting go.

    It doesn't happen if you gradually let off the power. Only if you do it suddenly (eg dipping clutch or revving).

    It sounds mechanical, very similar to when hydraulic power steering systems are straining (hence why I thought it might be steering).

    The noise happens in gear or out of gear with the car moving or static. Which I guess rules out Haldex.

    Possibly EGR or Turbo related? Presumably the turbo has spun up and needs to vent somewhere?

    Car is a TDi 170 Quattro on a 57 plate (manual box). It had a new turbo at about 35,000 miles and its done 60,000 now so I hope it isn't that.

    I scanned using VCDS at the weekend and there are no codes at all recorded.

    One final thing, maybe a coincidence but the car was low on oil the other day (it needed about 500ml put in). I know TDi's can go through phases of being thirsty but I mention it just in case. I do check every 4-6 weeks but then again my old Golf 140 used to go through the same kind of oil burning phases.

    Cheers.
    Last edited: Dec 18, 2013
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  2. c_w
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    c_w Active Member

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    Might be the flywheel ?
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  3. chinnyhill10
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    chinnyhill10 Member

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    Don't think so as there is no vibration and it's only on lift off while going past a certain RPM range.
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  4. max69vk
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    max69vk Active Member

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    It could still be a resonant noise create by the flywheel vibrations as when it is under load any 'slack' in the DMF will be taken up not allowing it to move & vibrate. If as you say you're not actually feeling any vibration it could be the clutch release bearing making the hum.
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  5. chinnyhill10
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    chinnyhill10 Member

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    I'll try and record the noise and put it on here.
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  6. chinnyhill10
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    chinnyhill10 Member

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    Recorded it but even but even with pro recording equipment not sure it picked it up above the general engine noise.

    With the bonnet up it seems to almost co-incide with the turbo spooling down as it drops below 2k rpm. You can hear the turbo working, as it spools down the noise begins.

    I've linked a audio file anyway just in case someone can hear anything amiss - https://dl.dropboxusercontent.com/u/1839679/1/STE-001.mp3

    Listening again, that spooling down noise could be it. Sounds different in the cabin but then again there's so much sound proofing.
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  7. chinnyhill10
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    chinnyhill10 Member

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    Just watched a video on Youtube of an A4 owner who had a similar noise but far louder, and it was the diverter valve. Wondering if it might be that.
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