This is a new sound (as of today) my 18 week old pullet is making. Stress? Illness? Puberty?!

[Ohsrsly]

First, we need to figure out what you're dealing with. If it's bacterial it can be treated with antibiotics, if it's viral there might not be much you can do

Is there any white gunk in her beak or throat? Does she have any discharge around her eyes or nostrils? I would also get in touch with your state's poultry extension to see if they offer any kind of testing

Her breathing sounds normal now, and she’s up and moving about. I’m just still so worried!

 

[FrostRanger]

Her breathing sounds normal now, and she’s up and moving about. I’m just still so worried!

Perhaps something just got caught in her throat then. Keep a close eye on her for now

 

[Ohsrsly]

Perhaps something just got caught in her throat then. Keep a close eye on her for now

Update: I’ve heard her sneeze a couple more times now and the “purring” sound is intermittent still.

 

[Eggcessive]

I can’t see the picture or video. Purring or crackles when breathing could be rales, a sound of moisture in the airway. It could be mucus, and when she shakes her head or coughs, she make be clearing it. Watch for bubbles in either eye, swelling of eyelids, gasping, or thick nasal drainage. If you see those, you may need to get some antibiotics such as Tylosin powder for the water.

 

[MROO]

If she's through with the Corid treatment, give her a dose of a poultry multivitamin. Corid stops coccidiosis by starving it of the Vitamin B it needs to survive. The downside is that chickens need B, too. Replacing the missing vitamins won't treat that nasty cough, but it should help her fight whatever is causing it.