There's another kind of squeak, a very loud one. Pigs who are being chased or threatened by another pig squeak that way, and it means: "HELP DON'T HURT ME DON'T KILL ME LEAVE ME ALONE GO AWAY!!!!!" When a pigs squeaks frantically when you swoop it up, it basically means the same thing.
There's a soft chittering wheek they make when they are happy and go peacefully after their business, i.e. looking for food in the cage.
And they call you loud and clear when they want food, think you have food, think you SHOULD have food, or just because they feel like it so maybe their human slave gets up and marches into the kitchen to, guess what, get food.
And there's the - oh, wait. My pigs are wheeking. Maybe they want some parsley? *hurries off*