If they sound congested, perhaps they are congested.
Seriously, if we're going to be able to help you, we need a lot more information. A video with the sound they're making will help a lot.
What is your climate?
Are the chickens behaving sick?
Are they not eating?
Do they have bubbles in their eyes and a mucous discharge from their beaks?
Are they sneezing?
Are their crops emptying over night or are they full come morning?