If it sounds like Darth Vader breathing this would be my first thoughtS:-
Has it be recharged with gas lately, if not a recharge and replace the pressure (switch failure is common).
After that I would be looking at a compressor as they also have a history of failure around 70k mark.
Finally, it not uncommon for these faults not to show up when scanned with VCDS.
If you do a search for pressure switch or aircon you'll be able to find similar threads on this topic.