without actually being there to see for myself i can't tell for sure but the chirpping noise could be him bruxxing, its a rats way of purring. sometimes if they're sniffing at the same time it can sound a little werid. and the sneezing doesn't necessarily mean a sick rat if its in a new environment. rats see mostly with their sense of smell and earing as their eyesight is so poor you might as well say they're blind (close your eyes until you can only see out through your eyelashes then take away all color--that's pretty much what they can see. lighter eyed rats can see even less shadow). you know how a dog will sneeze out one smell to better getanother, well rats will do the same thing. if everything smells new then they'll sneeze out more smells to get others. this can go one for a few days actually, until the rat gets used to everything. now, having said that, not all rats will do this when brought to a new place all that much. it depends on the personality of the rat. that's not to say that he didn't bring something with him from the pet store though. but even that is pretty simple to treat with a bit of antibiotics. but if you have a health guarrantee then you can probably make the pet shop pay for the vet bills anyway, so long as the guarantee hasn't expired anyway. if its one of the types that its only good for exchange then i wouldn't bother with it. if this guy has an illness the rest are likely to have it too. but this is the gamble you make when you buy from a pet store. the rats there are not always properly cared for and the workers and owners aren't likely to have the time to handle and check each rat out and get vet care for the sick ones. they just have too many animals to do that with and most ig francise ones don't care wnough about that anyway when it comes to rats.