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Listen to the sounds your rabbit makes. Keep an ear out for pleasure sounds. Listen for warning noises. Respond quickly to sounds of fright.
Realize that your rabbit will probably be silent most of the time. As a prey species, rabbits have learned to be quiet in order to escape the notice of predators. Some rabbits do make an occasional noise, either when very happy, scared, or as a warning. This makes them very different from purring cats and barking dogs which use a whole vocal range to communicate. Rabbits have a limited repertoire of noises they make when they're happy. These include a very quiet purr, a soft clicking noise, and a very quiet grinding of the teeth noise, which means she is content.  These can be very quiet, so you'll need to pay close attention in order to hear the sounds. Your rabbit may softly grind her teeth or click while you're petting her in her favorite spot, like behind the ears or under the chin. Be aware teeth grinding can be a sign of distress, but in this case the noise is louder. The classic warning signal is a thump with a back leg to warn other members of the group of danger. A rabbit that feels threatened and is angry enough to attack may make a series of grunting or growling sounds. Likewise loud teeth-grinding is another warning sound of annoyance. Some rabbits, faced with a threat, will hiss. If you have two rabbits (one male and one female) and the male starts grunting, move him immediately, unless you are attempting to breed them. A grunting male is a sure sign that he is attempting to mate or breed with the female. Rabbits have a particularly penetrating and distressing scream. They reserve this for when they are extremely frightened or actually being attacked. If your rabbit screams, she may be in danger or pain.  If you notice your rabbit in pain, check for signs of obvious external injury and contact your rabbit's veterinarian immediately. Your rabbit may have a life threatening injury or internal issue that needs to be addressed right away. However, a rabbit's instinct as a prey species means when they are hurt they are most likely to hide or stay very still to avoid attracting attention, so do not rely on noise alone to alert you to discomfort.