A rapid, soft chattering sound from the teeth. Generally happens when bunny is being patted or cuddled. A very pleasing sound of contentment and trust. A sign of fear, nervousness or discomfort. Possibly a sign that bunny is in distress.
Observation is warranted.
Pay attention to posture and body language. Soft oinking sounds, especially those made by a doe, are often a part of the mating ritual, which can occur even with fixed bunnies. Soft, musical sort of sound, never used aggressively. A quiet and gentle approach is warranted. May occur while grooming you or another bunny, a sign of affection and trust.