Hi Chris.
I can tell you for sure that my Female frogs (Bell Frogs) do purr like you say. Its usually when another frog is climbing over them. Its a little bit like a croak but more soft and quiet. They don't do it allot but ive seen and heard it a few times. I thought that she was telling her male partner "not tonight dear"

Jade looks great by the way, she's filled out beautifully. How are they both reacting to each other?