All interchangeable lens cameras make some sort of click sound when taking a picture. Depending on the camera, there are a couple of reasons this occurs.
When the shutter button is pressed on a DSLR camera, the mirror flips up and the focal-plane shutter opens so light from the lens can be captured by the image sensor. After the elapsed exposure time, the mirror flips back down into its original position and the shutter closes. The mirror flipping up and down, as well as the shutter opening and closing, causes a brief clicking sound.
There is no moveable mirror in an SLT or NEX camera. In these cameras the click sound that is heard is caused only by the focal-plane shutter opening and closing.
IMPORTANT: Some newer-model SLT cameras incorporate an electronic front curtain shutter that helps reduce shutter noise. However, SLT cameras prior to 2011 do not have an electronic front curtain shutter.
NOTE: Since this sound is a result of a mechanical function being performed, it cannot be disabled.