Been back to Audi 4 times, they say they cant hear anything so I said please keep it on file as I don't want it to fail after warranty. It doesn't seem to be a damaging issue though, I think the heat from the engine is soaking into an incorrect part of the air con system. If you turn the air con off and back on again it sort of clears what sounds like a gas build up and then its fine for a while.