Just jumped out of my skin - hamster just made series of short, loud squeaks, apparently to himself. I just checked and it is definitely him, not the guineas. He's in his house/nest, seemingly rearranging his pantry. (His main hobby, now that old age has set in and he can no longer pretend to be spiderman.)
He is fairly outspoken, as Syrian hamsters go. He squeaked like mad when he hid behind the sofa and dh's rescue attempt nearly squashed the poor blighter and when he's wandering over my shoulders he sometimes sticks his snout in my ear and squeaks quietly, in a politely conversational sort of way.
Do they normally squeak to themselves? The guinea pigs aren't up to anything more than sleeping and eating hay, so it's not like there's a slanging match going on.