Mirror lockup is designed to reduce vibration. When you depress the shutter release halfway to get exposure and focus the mirror is down, allowing you to see the shot in the viewfinder. When you depress the button the rest of the way the mirror swings up to allow the sensor to get the shot. It swings quickly and will produce some vibration in the camera. With mirror lockup, which is normally only used when using a tripod, the first press of the button gets your settings and swings the mirror up. It takes a second push of the button for the photo to be taken. This lets you to allow time for the vibrations to stop before actually taking the shot. During Live View the mirror is also up so taking shots in this manner would eliminate that source of vibration, although, I have never noticed it to be much of an issue when shooting handheld.

Its not a problem nor a phenomenon. The noise you hear is the normal result of a dslr with a mirror and mechanical shutter box. Part of the sound is the result of the mirror moving up out of the way and back down after you take a picture. The other sound is the result of the curtains opening and closing as the picture is taken. Some high end dslrs have a quiet mode but it is only marginally quieter than normal and results in fewer frames per second. If you shoot in live view you will eliminate the sound of the mirror raising and lowering, since in live view it is already moved up. The main purpose of mirror lockup when using a viewfinder is to move the mirror up and out of the way, prior to the shutter opening, to reduce vibrations at slow shutter speeds. Point and shoot cameras are quieter because they don't have mirrors or mechanical shutter boxes.