Welcome to the forum you'll find the members on here are a friendly and helpful bunch 🙂
Glad you got the problem sorted.
I'd recommend you try eBay for the mirror.
If battery is in engine bay then the drains for the battery location could be blocked. First thing I'd do is remove the skuttle panel near the bulkhead to see if you can see any trapped leaves etc. It's the black plastic with vents either side.
https://www.audi-sport.net/xf/threads/342829/
It could be an issue with the hose itself letting additional air in where it joins. Under load the additional air could be being drawn through due to the pipe not being as tight due to age.