Rat rooms and cages need to have good ventilation due to the possible build up of amonia gasses. Do you cages have proper ventilation and are any of them in a position which could cause a dead zone of no air movement. Also do you have any kind of exhaust fan set up. Not sure how many rats we are talking about her but if you have a nice size colony, 30 or more rats you need to have at least an open window and you start getting up to 50 or more you need some kind of forced air ventilation. Odds are against this being the problem since so few seem to be affected, but it might be!