I think you should supervise closely to see if you can get a better read on the situation. It sounds like your bun is telling your 6 year old that he feels threatened and to “back off”, so you should teach your child that when bun makes that noise, it means to stop petting. This is usually more likely to happen if the petting is happening within the bun’s cage or pen, as they can feel more trapped and territorial, so petting when the bun is out in a open space (and has room to leave if they want to) usually helps the bun feel safer.
There are also “clucking” or “oinking” noises that are happy noises, but those usually go along with more relaxed body language, and don’t really sound like a grunt or growl.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.