2006 3.0 TDI Quattro (233) - Humming/Buzz noise under accelleration - PLEASE HELP!!!

[Lion Heart]

Hi guys

Need help!! The car is a 2006 3.0 TDI Quattro A6 Avant, tiptronic auto.

When under heavy accelleration (noticed mostly when moving away quickly at roundabouts, traffic lights etc - mostly in the lower gears) there is a very loud and noticeable noise that lasts until it's up to speed and then stops.

It's a loud kind of buzzing noise? Maybe humming is more description. It's a kind of 'BBRRRRRRRRRRR' noise, and it can last a good few seconds at a time.

It's most noticeable pulling away from stationary quickly (heavy accelleration), but can also do it at speed if you put your foot down.

The noise seems to be independant of the gearbox (note geabox works fine, all changes good etc), but hard to tell because of the course the revs change when the gear changes. You can force it to make the noise sometimes if you drop a gear (which pushes the revs up high again). But the noise is seperate to the revs of the engine if that makes sense.

The car drives beautifully otherwise. Cruises absolutely perfectly, great on the motorway etc.

Any idea what on Earth this annoying (and worrying!!) noise might be???

 

[fishyfish]

Based on absolutely no proper mechanical knowledge (just my own knowledge-base as owner of same setup), my suggestions would be:
- swirl flap motors (not very likely, but possibly)
- turbo variable-geometry/nozzle actuator (maybe something loose and/or sticking)
- turbo bearing in not so great shape (hopefully not this!)

The way you describe it, it seems to be when you vary the load on the engine, and seems to point to being turbo related (i.e. kinda linked to engine revs, but not directly).

That said, internet advice is worth precisely squat, so I'd say take it to your nearest VAG indie garage and get them to have a listen, as its pretty much impossible to know without hearing/experiencing the sound in person.

 

[Lion Heart]

That's interesting. It's quite physical also along with the noise. It gives of a reasonable bit of vibration alongside the noise if that helps diagnosis.

 

[Lion Heart]

Hi Guys

Wanted to clarify and update. Maybe my description of it being a humming or buzzing noise threw you. It's maybe more of an outright vibration noise. That's probably a more apt description.

Ok, here's the update:

Took the car to my local Audi main dealer. I thought they'd know the fault pretty much immediately as I'm sure it's not the first car in history to have this fault. I thought an hours big fee is worth a definitive answer. £480 + VAT later and they have given me their opinion:

They say it is (possibly!) the prop shaft. They have tried moving the prop shaft around one bolt hole at a time to try and get it more balanced. They said that it's improved the vibration slightly but not cured it, but having driven the car I don't think it has improved.

The replacement prop shaft is big money, so they recommended getting it balanced by a specialist.

This being said, I'm not sold that it's the prop shaft. You can feel the vibration when accelerating hard, but you can also feel it slightly (though much less) when braking to a stop (i.e. under load, but slowing). If it were the prop shaft being out of balance it would not do it at all at very low speeds?????

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INTERESTING UPDATE:

Driving back from Audi I got caught in heavy stop-start traffic for 20-30 minutes, which was actually a blessing...

I noted then when stopping and starting every now and again (say 1 in 5 average) you would come to a completely stop, then hear four audible 'clicks' coming from the front of the car, followed by a very violent rattle for 1-2 seconds which distinctly appears to come up the pillar between the windscreen and side window on the drivers side - and it so violent it rattles the trim on that pillar.

I'm not saying it comes up the pillar, but something in that area rattles very hard and makes itself apparent from within the car by rattling that pillar.

But weirdly there are always several noticeable 'clicks' before the rattle.

Any ideas????