Change the Haldex fluid and clean the filter screen of the pump. If your Haldex is the older "Gen-4" version, it also has a filter cartridge that should also be changed. The newer Gen-5 Haldex does not have the filter cartridge, only the pump screen needs to be cleaned.
Also change the rear differential gear oil.
Also change the front "Bevel Box" gear oil.
Also change the automatic (or DSG) transmission fluid and filter. If manual transmission, change the gear oil (no filter).
Use OEM VW/Audi fluids. VW/Audi does not sell a replacement Haldex filter, you will need to buy that from someone else.