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 deep '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???