I had many rats, one of my very favorite died suddenly from a similar sounding problem.
I found it easier to google the information rather than figure out how to write this out, but this should be helpful.
If your rat is sneezing more than usual, listen closely to his breathing. If the nasal passages and chest sound clear, keep a very close watch on him, but it shouldn't be necessary to start treatment unless he starts acting more tired than usual, or stops eating or drinking. Allergies and dust are also possible causes of sneezing.
If there is a clicking sound from the sinuses, he is congested. This could progress further, so treatment would be a good precaution at this point.
If there is a gurgling, congested sound from the chest, you have a sick rat. Get him to a vet immediately! Rats succumb to these infections with amazing swiftness. A rat with no symptoms one day can have full blown pneumonia the next. This is partially because of their high metabolic rate, and because there is often a secondary infection caused by a different bacteria or virus.
Does your rat have red stuff on his nose, or around his eyes? This is called porphyrin. Porphyrin is a red mucous that is produced by a gland behind the eye (the Harderian Gland) which dries the color of blood. It's often mistaken for dried blood by people unfamiliar with rats. Porphyrin staining is often associated with mycoplasma infections because the infection puts stress on the rat, and stress will cause the discharge of porphyrin from the eyes and nose. This doesn't always occur when a rat is suffering from an infection. If you have a really laid-back rat, he may not get stressed out! There is also the possibility that your rat is being stressed by something else.
If your rat's fur is standing somewhat on end making him look puffy (a bit like a cat when it's been treed by a dog) and he's hunched up, he's feeling sick, uncomfortable, possibly in pain. Get him to a vet immediately!
From:
Rat & Mouse Gazette: Respiratory Infections in Pet Rats