Like Pinkie said, myco is carried in nearly all rats. What ends up making a rat sick is when a secondary infection creeps in. Some of those infections are pretty low level like in my Cadoo and Cosmo. They were sneezing a bit and definitely crackly breathing (off/on). But oddly enough no polyporphrin. They still played, ate, and had no trouble breathing. Both just finished up ABs. They are 12wks old. Our rat, Inky, at 19mos old, within 24hrs, went from a healthy happy girlie to deathly ill. She was in heart failure and some kind of URI. It was obviously a vicious infection. She had no polyporphrin either. But definitely in bad shape. We took her to the vet. She was given an injection of Lasix and some ABs, but the infection was just too much for her. She died the following morning.
If you think your babes are crackling/sneezing too much and if you can hear noise in their lungs when you do the ole ear to chest test, you may want to have a vet check them out.
I'll be the first to say that I tried to hold off the vet with Cadoo. When it became obvious that the crackling was in his chest, I hauled him off to the vet. With Inky there was zero doubt about her being ill, she went right away.
I hope your babes are okay.