Although turtles like most other reptiles do not have vocal chords, they are still able to make several noises both voluntarily and involuntarily. For instance, turtles have been known to make involuntary noises such as hissing when they jerk their heads into their shells. This happens because it forces air in the lungs to escape in order to make more room for the turtle.
As far as making noise voluntarily goes, it is quite common for a turtle to do so during battle, courtship, mating and egg-laying. Most turtle noises are hisses; however turtles have also been known to make noises that include grunting, hooting and clucking.
What is even more interesting as far as sound goes is that turtles do not have outer ears. They have inner ear structures but do not necessarily respond to noise. Rather, they respond quickly to vibrations that they pick up through their shells or limbs.
It is common for turtles to hiss when they feel startled, threatened, or bothered. They may also hiss when being territorial towards other turtles, and at predators. Some pet owners have even said that their turtles sometimes make a moaning sound if they are hungry and want to be fed. This moaning noise has been described by some pet owners similar to that of a dog whimpering in kind of a high pitched way. Imagine something along these lines “ummmm, ummmm.” Other pet owners have described similar sounds as being due to respiratory infections. It is always a good idea to double check with a veterinarian to make sure that this isn’t the case with your turtle.
Turtles should never really make any noise when breathing. If this occurs it most likely means that your turtle is ill with some kind of airway or respiratory infection or illness. In this case a veterinarian that is knowledgeable of reptiles should be consulted immediately.