You described it very well, I know exactly what you mean.
Velvet seems to make that sound to announce herself when she enters a room or just to get our attention. Kind of like "look at me!".
I used to have a cat that never meowed, I don't think she was capable. All she made was that noise.
I just love it, I think it is so cute!!
have you noticed how some of those "chirps" end on an up-inflection, and some on a down?
Yup. An upward inflection means Assumpta wants something, and a downward one means "Here I am." I think this is one of the things that cats learn to communicate with their humans, because she didn't do that when she first came here, everything was a statement, never a question.