This is singing, greyhound-style. It's something like a wolf's howl but without the yelps most wolves and coyotes make. Wolves sing "how-how-howrooo!" but the sweet greyhound just goes "rooo" Often, they will do this at a certain time of the day (the midafternoon or sunset) If a greyhound hears another rooing she'll join in. At the large kennels kept at greyhound racing tracks there is often a certain hour of the day when 1000s of dogs will join together in a group "roo" that is possibly one the most chilling and unearthly things I've ever heard. If you have a pet greyhound and they have no greyhound friends to roo with consider rooing yourself (your dog will join in) it's therapeutic.