Difficult to do a proper diagnosis without seeing / scanning the car but it sounds like the throttle body.
Had a friend with similar symptoms (assuming you have a V6), the noise was coming from the throttle body (just behind NSF headlight) but when removing the engine cover the VNT and EGR actuators were moving to and fro randomly - which was adding to the tapping noise.
When I scanned the car I got the following faults :
004203 - Throttle Valve: Stuck or difficult to move
P106B - 001 - Upper Limit Exceeded - Intermittent
005504 - Throttle Actuator (Bank 1): Malfunction
P1580 - 008 - Implausible Signal
When I removed the throttle body the flap was seized and the internal plastic gears stripped so I had to replace the throttle body.
I hope your problem isn't this as the part alone is in the region of £300 
Where are you based ?, maybe you can find somebody local who can scan it for you.
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2003 D3 A8 4.2 Atlas Grey -
Factory: Adaptive lights with coming home function, 19" wheels, Solar Sunroof, Bose, Nav, TV, Paddle shift sports wheel, sports suspension, sports seats, rear seats with electric lumbar adjustment, heated seats and climate all round, ski hatch, int light pack, auto dip/fold mirrors, voice command, cordless phone, wooden gear knob, towbar, power boot lid
After: LPG, AMI with HDD
Pending: DVB-T Tuner & fixing niggles
Last edited by Godders; 19th August 2011 at 09:11 AM.
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