Hi! I just checked mine on the 5Dmk3 and yes, it's pretty noisy with clicks, a high pitched whine when making major focus changes, and little juddering sounds when making fine adjustments before focus lock. This is a "STM" or Stepping Motor lens, and from what I read it was designed to go with the new T4i which has continuous autofocus while shooting video. Thing is, I *never* use autofocus when shooting video, always manual focus. You will find if you switch to manual that it has a smooth, damped focus ring in manual that is really easy to turn and very quiet. When shooting stills, the focus noise is not an issue unless you're worried about startling wildlife or distracting subjects like children. When shooting video, it's better to use your Canon camera's digital zoom to punch in to 5x or 10x, focus manually, then go back to normal and shoot. With a 40mm lens, if you're shooting at f/4.0 or more, focus shouldn't be too much of a problem especially on a crop sensor camera. But if you are using it on a T4i to shoot video with continuous autofocus, yes it's going to be noisy and probably your camera will pick up the noise as you're shooting. The only way to avoid this is to use an off-camera microphone, a wireless mic setup, or even a dual-system sound method where you record with a mic to an external recorder like a Zoom H4n and then sync the audio when you edit the video. A simple hand clap is all you need to do either right after you start recording on both the camera and the audio recorder or right before you stop the clip (also known as a "tail slate"). Bottom line: the lens is sharp as a tack and easy and silent to focus manually for video, but it's definitely a "talker" if you're using autofocus.
It's not silent. It's quiet but not silent. Quieter than the 50mm f/1.8.
If you want SILENT, try the 18-135 STM and you'll know silent. The 40 STM is a great lens but not silent. If you're shooting video, look toward a USM LENS for fast focus and quiet operation (i.e., 28mm f/1.8).