Lower-end cameras, such as all camera phones and most P&S cameras, can be completely silent when they take a picture because they don’t have a physical shutter. DSLRs have both a mirror and a shutter that needs to open and close with every picture you take, and that process makes a distinct thud or clanking sound. If you’re shooting portraits or landscapes, the sound will make no difference to you whatsoever. However, if you’re a photojournalist or a wedding photographer, or if you shoot candids or wildlife, the mirror/shutter noise can be extremely important to you. Imagine needing to photograph a funeral without disturbing the grieving, or waiting hours for a fox to come out of its den, only to have it startled by the noise from your first shot. Some newer DSLRs feature quiet or “silent” modes (which typically aren’t really silent). On some bodies, the mode is only available when shooting in live view mode, so you can’t be looking through the viewfinder when you use it. However, many new bodies reduce sound when using the viewfinder by moving the mirror a bit slower, reducing the noise it makes as it bangs open or closed. Unfortunately, nobody seems to document the sound levels of different cameras in any standardized way. However, if you search the web for specific models and the words shutter and sound, you can often find users who have done testing on their own. If low sound levels are really important to you, you might look instead to mirrorless cameras. If your DSLR really must be silent, consider buying a camera muzzle or a sound blimp from http://www.soundblimp.com/.