The body language accompanying Belle's soft grunting signifies to me that she is greatly enjoying being petted and stroked by you. Body language is very important when trying to clarify a Rabbit's vocal message. For example, when a rabbit grunts while appearing tense or uncomfortable, that is a warning signal to move away and is the prelude to a growl. Grunting is also heard when a rabbit is circling around your feet, which is their way of flirting.