The harmless versions, for completeness, because they do exist. Startled mid-meal, since they are prey animals and a hand appearing over the cage while they are concentrating gets a squeak. And guarding food, which they are famously serious about.
The difference is whether it happens when nothing is happening. Reach toward the bowl and get a squeak, that is her telling you off. Squeak on the bite with nobody near her, that is her mouth.