In my 2001 Audi TT since 2 years looks like some sort of foam material inside the AC ducts is desintegrating, maybe the FL heat ? Debris coming our of all 4 vents. Tried everything even sticking a vacuum in the vents and closing with metallic mesh. Nocthing works. Stuff still coming out.
Any advice, please ??? cant drive the car, i am choking !

 

Have a similar issue with my 02 A6. Doesn't sound as bad as yours though. Hoping your answer is my answer as well.