You never mentioned if the noise was inside or outside the car

so here are a few suggestions

if the noise is inside the car check the rubber boot/gaiter on the steering shaft, it sits just below the steering wheel, if that is your problem try straying it with Silicon spray or rubber lubricant, DO NOT use grease or    WD-40           

If the noise is outside it is more than likely the rubber mounts on top of the shock absorbersthey get dry and cause more of a groan, the easiest way to do this repair is to JUST LOOSEN the 3 bolts on top of the shock absorber tower under the bonnet (Hood) and use the car jack to raise the body just enough to allow you to spray the mounts with silicon spray.

Don't forget to re-tighten the bolts after you have completed the repairs.

You may also want to check to see if you have a mouse in the ashtray   

Let me know how you get on

Thomas :) 

Thanks Brit (pommy!)

Ran it by an audi dealer where a salesman told me that that noise is a normal quattro thing.  The diff is compensating for the forces whilst turning at slow speed.... Would rather had heared this from an Audi  mechanic than salesman!  What do you think (A6) quattro owners???  Do you all get this rubbing/thrumming noise?

Anita,

2 things

1) I'm from England NOT Austraila therefore I am NOT a pommy

2) the advise the salesman gave you is BULLS***

Audi diffs do not compensate for anything that is standard salesman verbage, I still say the fault is what I suggested earlier, however I could be wrong seeing as I haven't heard the exact noise, but I do not it's not a common Audi A6 quirk 

2 things, 1 lube lower boot or 2, in alot of cases the steering colum ujoint needs to be lubricated and "sometimes" loosened/lubbed and re tightened.

if it is still there, check your tie rod ends, the boots tear and you get a groan from the dry rod end, to test, spray some silicone lube in the tir rod end "Boot"  IF it is torn.  If noise goes away, replace tei rods and get an alignment.