Squeaking just started

Question
Our rat, Sophie, is ~2 1/2 years old. Just in the last week, she has started a squeaking noise.When she's asleep, you can hear her inher box. When she's on my shoulder, she chitters as usual, except that it now makes a squeaking noise. I'm not sure it's strictly breathing, because it stops occasionally, for instance when she's eating. She acts just the same as always. Only other indication is that her right eye is maybe 3/4 open. She's not sneezing, coughing, etc. Any idea what this could be, and if it's something that I should have treated?

Answer
SHe has a respiratory infection from the sounds of it. Getting it treated properly by the right vet that knows about rats is important. She should be given baytril for at least 21 days. If you need a vet please let me know and I will happily give you some names!