It's called stridor, and it usually stems from a small blockage in the airway. A lot of the time, it self corrects overnight, but it could be more serious and needs to be fished out with tweezers. Wrap her up securely in a towel to confine wings and feet. Then have someone hold her while you pry open her beak wide and look into her throat with a strong flashlight.
You may see a hair wrapped around her tongue or a blade of grass stuck at the back of her throat or even some yellow gunk that could signal the beginning of an infection. The throat should be pink and clear.
Another thing it could be, but less likely, is a crop disorder. Feel her crop now and again in the morning first thing before she eats. If it's full, she has a crop disorder that needs treatment.