A lot of animals hiss as a way of expressing emotions such as fear, anger and anxiety. Cats are a perfect example. Turtles, too, are capable of producing hissing sounds, even though they do not have vocal cords. In the case of turtles, however, the hissing sounds you hear are not of their own volition.
Fear Rather Than Anger
Hissing in turtles generally isn't an indication of belligerence and hostility, but rather of pure fear -- poor things. If your pet turtle hisses, he's not trying to alarm you or warn you, as the action is simply involuntary on his part. When turtles feel scared for whatever reason, they usually rapidly force their heads back into their shells. This action is not something they can control; it just happens, perhaps similar to the way a person might cry out in the middle of a daunting situation.
Why Does the Hissing Sound Occur, Then?
Since all turtles are vocal cord-free, the notion of them hissing might seem rather bizarre and implausible. However, the hissing sound you hear when a turtle retreats back into his shell is actually just air being expelled. In order to fit inside their shells properly, turtles' lungs need to emit air quickly. This action produces the conspicuous hissing noise you hear.
Dual Purposes of Turtle "Hissing"
In turtles, hissing sounds can serve a dual purpose, according to the Friends of the WNC organization in Asheville, N.C. Not only does hissing potentially scare off sources of harm, it also allows turtles to feel a lot more comfortable within the confines of their shells -- not too shabby at all.
Turtles and Petting
If you notice that your pet turtle "hisses" any time you make an attempt to pet him, for instance, it may be a sign he doesn't like it or simply isn't ready for it yet. Other indications that your pet turtle doesn't really appreciate being approached include running away every time you're close to him, his constant dodging of your touch and an overall tenseness to his body.