Are my chickens getting sick?

Help! Not sure if my chickens are getting sick or not. I have 9 chickens that are about 3 months old. We've had 7 of them since they were a week old, the other 2 are "new." My husband bought them at a local chicken exchange sale, we isolated the two newcomers for the recommended 2 weeks. They seem just fine, healthy and all are beginning to get along. But, two of my "original" flock chickens, Charlotte, my large, fluffy Brahma, and Oreo, our Bantam Rooster seem to have developed a "cold." They appear fine, eating, running around, doing their free range thing and look healthy, however, their breathing sounds like they have a cold. I've just noticed this for the past 2 -3 days, none of my other chickens so far sound this way, but I worry that they could be getting sick. We are new to this whole chicken farming experience, BUT we LOVE, LOVE our babies, so any suggestions, offers of help, etc.. would be greatly appreciated! Thank you in advance!