Respiratory disease of chickens?

[luludaxia]

The week before last, I found one of my pullets (6 months old, beginning to lay) was sick. She's breathing with open mouth, with a rattling sound, sometimes she made a loud noise which to me sounded like coughing. One of her eyes are closed. She's not eating and it seemed she couldn't stand up. But she had no discharge in her nose or eyes, eye not swollen. No bad odor from her nostril. And she does not look lethargic. I kept her in a separate cage and she became better next day although still panting. Then she recovered. Now she looks normal and eating a lot everyday, even her eye is opened. I didn't give her any medicine in the whole process.
Then this week I found a hen in my barn fall ill with the same mouth opening breathing with rattling sound. The two chickens have not direct contact with each other. All other chickens look fine so far. So what disease could this be?

 

[luludaxia]

At that night when I separate the pullet from others I heard another hen in the same brooder have rattling noise when she breathed. But she was otherwise fine, she's eating. And the next day the noise disappeared.