The hissing noise that you hear is not defensive unless their head is completely visible and their mouth is wide open as if they're going to bite. Red Ear Sliders, along with Painted turtles and some others will often attempt to defend themselves by biting. Box turtles almost never do.
If the turtle's head and other limbs are pulled in, the hissing noise is very likely from the turtle rapidly expelling air from it's lungs which takes place when they quickly pull their limbs in, usually due to being startled, such as when picked up.
that is normal until they get to know you..and even then if you startle them they will hiss, HOWEVER you will also hear a hisswhen they suddenly flinch in their shell