If its a rattling type sound it very well may be CRD. Once they have it, the symptoms may go away or the bird may eventually kill over. Regardless, once they contract it, it stays with them for life and is even passed on to offspring as a carrier. Even though I'm not a big advocater of chemical usage, I have found that placing a capful of bleach in their water (I use one capful per 5 gallons) does wonders. It not only kills any mosquito larvae that may be present, but anything else that might be lingering around. My hens dont seem to mind, just dont get carried away with it.