I'd love to know what they are thinking. I hum more than I sing but when I sing, which I know is terrible but I still like to do it when no one else is around, the one dog gets really excited and crazy. I don't believe it is necessarily play, but could very well be to say, "Make it stop! It's driving me nuts." My poor, captured audience is abused. So I try to stick with humming, then I have their undivided, calm attention.