I've seen some posts about hissing. My A4 2.8Q has always hissed for a few minutes when you turn off the engine. I recently had to have the AirCon compressor & dryer replaced as it had been slowly leaking since I've had the car and no one could locate the leak (Audi are useless on AirCon, £90 a time for re-gassing). I thought the hissing might stop once I'd spent £600 on the ungrateful AirCon system, but it's still there. Any ideas? Or is this normal. It does actually sound quite good, like it is slowly relaxing after a good thrashing, but it can't be right can it?