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I didn't realise you'd had him that long, it does seem odd that he would suddenly begin making noises after that length of time but if he seems fine otherwise then I don't think it's a cause for concern really.
Some can be a bit prone to nibbling or biting after taking treats, most usually grow out of it but if he hasn't try not to respond at all if he does manage a nibble but also try to avoid it by either gently removing your hand or putting him down if you hold him before he gets a chance , if he keeps doing it you could try just putting treats down in front of him when he comes to you for a while so he gets over the fingers might be food or I might get more if I bite behaviour!