Amber, Clicky Breathing and mucus around the eyes

Crouched down watching & chatting to my hens just before shutting them up tonight, I spotted Amber's (amber star) cheeks were going concave each time she breathed in, and bulging out when she breathed out. There was a slight click between breaths, and occasionally, a few bubbles would appear in the inner corner of one or other eye.

I would suspect some sort of cold virus, but her nostrils are completely clear - no discharge or snot and she seems otherwise absolutely fine.

I have had hens for 8 years, and have had snotty chickens that have recovered after a few days. I've never noticed this cheek movement and bubbly eyes before.

I have some Tylan so I'm going to give amber a 7 day course.