It sounds like an affectionate noise to me. Pidgey makes a similar sound when I'm near the cage, or when I hold him. If she's by herself, she probably formed a very strong attachment to you. Doves tend to do that, since they're social birds. The same thing happened to Pidgey after his cage mate of 18 years, Spice, died. He was very lonely and started bonding with me and exhibiting new behaviors. For example, he started moaning when I held him, and preening whenever he sees me brush my hair in front of his cage. I hope this helps.