I have been told by one person who breeds rats that they do hiccup and that the chirping sound is because it feels uncomfortable. I talked to a vet and he stated that they could possibly have a respiratory problem. He said they do hiccup but that it's normally silent. He also said that, in general, rats are quiet, and if they make chirping or grinding noises, it could be an indication there is something wrong. Your vet is right regarding the chirping noises.
Rats do get hiccups and they are indeed silent.
Chirping noises could be an indication of a possible current respiratory problem, or even scar tissue from a previous respiratory infection. Grinding noises, like when they're grinding their teeth, are completely normal, but for the most part, rats are extremely quiet. Grunts, chortles, and chirps are frequently signs of impending or current respiratory infections. A. Small amounts of sneezing are normal, but anything beyond that is not. If you are not knowledgable or experienced enough in dealing with rat respiratory problems to treat them yourself, you should take them to the vet if you have any doubts and let your vet be the judge of your rat s condition.