Most chicken respiratory diseases aren't viral; one of the commonest (mycoplasma) is bacterial. If a bird develops only mild gurgly symptoms, it could well be an underlying case of mycoplasma gallisepticum that the bird has been harbouring since hatch. Stress causes the bird to show symptoms. (On the other hand it could be viral, or it could be something caught in the throat.)
I agree that it's not a good idea to treat all birds with Tylan or other antibiotics -- apart from anything else that's a waste of medicine. It would be wise to separate this bird and treat her immediately (if that's your choice) or at the very least give TLC and see what happens. However remember that, if it is a genuine respiratory disease, treated birds remain carriers, and will quite likely get sick next time they're stressed. They may also produce infected eggs.
Whatever the case, lowering yard stress can help reduce the number of birds who show symptoms -- good quality food, fresh water, clean roost areas, no moving chickens from pen to pen or acquiring new ones. Unfortunately once a respiratory disease is there in the flock, it tends to stay there, no matter how often you medicate (or how clean your pens). This is why some people choose antibiotics as a revolving door approach while others take the drastic approach of a total cull, cleanout and start again.
Good luck, whatever you do.
Erica