I've noticed one of my guinea pigs doesn't react to noise anymore. If he's lying there asleep and I walk over to him and say his name, he doesn't even twitch. All my other pigs do, it's just him. He could be fast asleep and I could in and clap my hands behind him and he doesn't move a muscle. If he spots me out of the corner of his eye he'll jump in fright but being a guinea pig, he should have heard me coming long before he see's me.
He doesn't react to my rather ridiculous cry of "look what mummy has for you!" like all my other pigs know what this means and he used to - but now it's like I've not even spoken.
I could rattle a carrot bag behind his head and he just carries on staring directly ahead and will occasionally itch himself etc but does not react to the noise of the bag at all.
He never used to be like this. He used to recognise my voice straight away and come running. He was always the first to the front when the carrot bag was rattled. He's also suddenly less tame, he won't allow his head to be stroked unless he's in a REALLY good mood and he HATES being picked up now - he used to be my friendliest pig! 
what has happened to him?