Sounds like resonant vibration induced from somewhere into something. Whilst that's not very helpful in one way it might help in thinking about what's causing the noise. You say that it can occur on idle and under acceleration so perhaps as the cylinder firing/exhaust passes through various rpm it may cause resonance at various harmonics. e.g. at idle of 850rpm the engine is firing 4 cylinders per revolution (so far as i understand), therefore the firing rate per second is 56.6 p/sec. If that rpm causes induced vibration in something (e.g. exhaust components/ broken baffles etc), multiples of the same frequency (harmonics) can do the same which is perhaps why it occurs at different speeds/rpms. I might of course be talking out of my own exhaust and and its caused by a loose scewdriver rolling around in the boot but i'm trying to think out of the box..... 
I'll get my coat.....
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2002 D2 S8 – Ming Blue, Valcona leather, Vavona wood insert, solar sunroof (to be fitted), Heated rear seats, extended leather pack, 18” Avus, ski hatch, Bose, auto dim rear view mirror, rear blind.
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