I have a similar issue. It started with a runny nose and progressed to yellow mucous coming out of my pullets nostrils. She was lethargic, but eating and drinking until this afternoon. We've cleaned her nostrils of mucous, treated with VetRx, gave probiotics and Duramycin in water. She smells like a dead bird.
She perked up a bit with fluids, but I'm wired as she sounds gurgly, like she's got mucous in the back of her throat. I don't hear any chest rattles.
She's in the house in a cage,now, away from the rest of the flock.
When I went out tonight, there was a second pullet with dried mucous on her nostrils. Brought her in and cleaned her up. Treated with VetRx. She's pretty feisty, which I hope is a good sign. She smells too, but not as bad as the first one.
I keep the coop cleaned of pop and wet straw every other day. I have two waterers and two hanging poultry feeders. No moldy feed that I can see.
Run is 25 x 15. Coop is 4' x4' x4', with two roosting bar heights. Small 12" box fan blows through coop all the time.
Not sure what else I can do for her or the rest of the flock. There are no vets close by that treat chickens.