This is the sound young audiences, influenced by America perhaps, are heard to make in an attempt to vocalise their opinion that something is tickling their romantic fancy! Woo! I think it's the same as 'Ooh! Ah!' But when they do it three times, it's a show of appreciation like 'Hoorah!' Sometimes even the stones cry out! Wouldn't you if you had 300,000 people stepping on you each year?