Apologies if this is old news, I searched the forum and saw no reference to my weekend discovery so thought I'd share my findings here:
I've recently bought a one-owner A4 3.0TDi Quattro S-Line Avant with 50k miles and full Audi history from new - all at the supplying dealer. The car is terrific but, being a compulsive meddler, I set to work at the weekend starting with a headlamp bulb upgrade. To get access to the little buggers, I removed the air intake housing complete with intake duct/snorkel and, despite the full Audi history, (the last being only 1500 miles ago), I found the gauze pre-filter in the duct to be choked with a solid 3" thick wedge of leaves, straw, dust, gravel, feathers - you name it, it was in there!! In short, there has evidently been zero air flow through that duct for a matter of years I would guess. Amazingly, I didn't detect an obvious lack of power when driving it from sunny Somerset to snowy Scotland and it averaged almost 42mpg at 80mph (DIS). I haven't had the opportunity to drive it properly since cleaning out the duct, but it can only help..!