I second Gavin for that one,

Secondary Air Intake System, or SAI for short. Loves to sound like a jet engine once the Combination valve or N112 valve start to fail.
Shops will say replace every component(and charge you so so much), so everyone here will tell you to throw it out and get it code/resistor it out. Will save you in the long run. If you remove it without coding, car will run terribly.

VW is anyway getting ****ed for their "upstream" tech, which I understand as these SAI systems(blowing fresh air through the exhaust) so will see how that will go.

Good luck!