We have two hens who do that. It is their breathing. It doesn't always seem like that, but that's what it is. It's a weird crackling noise (just like bronchitis can sometimes get in people). And it gets worse during hot, dry, dusty weather. Both have been that way for months. Neither one has shown any other symptoms. They both seem happy as can be and run all day with the others. And they are free range and run all over the place. None of the others have shown the symptoms. It is a bit more noticeable at night. It could be dust...or possibly something else which is taking a long time to materialize. I am going to try some organic apple cider vinegar in the water, and just watch. There are so many things it could be, we decided not to panic about it yet.