If you're shooting in a very quiet environment, or trying to be as surreptitious as possible for a candid shot, the click of an SLR shutter can be as loud as a gunshot. So what do you do? You can use a quiet mode if your camera has one, but those don't always make much of a difference, or you can spend a fortune on a muffling rig. Or, with a bit of know-how, you can build your own.
via Imaging Resource News Page http://www.imaging-resource.com/news/2013/06/06/How-to-build-your-own-soundproof-Sound-Blimp-SLR-case-for-under-100