Some chins are just talkative. I have 2 that make all sorts of noises when you interact with them and one that seems to just talk to himself. We named one Keck for the noises she makes at us. She'll also make little crying squeak noises occasionally when my husband reaches through the bars to touch her without having a treat in his hand. She's friendly, not stressed, not scared, although she doesn't really like being handled. She's young though and might become more tolerant with age and handling. If there are no other symptoms and he is only doing it when you try to pet him and not sitting there squeaking and/or grinding teeth which indicates pain then he may just be a vocal individual. You should watch closely for possible problems though. It would be a good idea to get a kitchen or postal scale to weigh him at least weekly for now. A drop in weight is often the first indicator of a health issue. Setting a bowl on the scale and the chin in it makes it much easier to get a weight on these very active critters.