Originally Posted by
S.o.S.
Welcome to the Cabin EnglishMartyn. The answer is contained in a thread in the Forum FAQ.
This is often called 'the barking dog' as it sounds like a dog is barking in the hold area. It is the Power Transfer Unit and 100% normal on Airbus aircraft. There are a number of places to read up about it. Usually heard on the ground just after start up and before shut down.
My bad, forgot to look. That sounds plausible, thank you.