Samsung states - A buzzing sound from your plasma TV is normal. It is caused by the electrical charges used to create the images on the screen. The buzzing, however, should not be so loud that you can hear it when the volume on the TV is set at normal levels.It is also important to determine the type of buzzing sound. A sound like a ground hum is completely normal. This buzzing can originate near the power cord connection or from the SMPS board position on the upper back of the TV. The buzz will be more audible as contrast increases on the screen, most notably when displaying an all white screen. If the buzzing sound is loud, you may have the Contrast set too high. Lower the contrast to reduce or eliminate the buzzing.You may also have loud buzzing if the rear of your Plasma TV is too close to a wall or other hard surface. Try moving your TV away from the wall until the buzzing is minimized. 
If the buzzing is at a higher frequency and resembles a crackling sound (like the sound of Velcro being pulled apart), and originates from the upper middle back of the TV, you likely have a failing SMPS board. Place a support call with Samsung to have the SMPS board replaced.FP-T5084
Have a known buzzing problem with a very simple fix.If the buzzing is coming from the left or right sides of the rear and the volume can be changed by pressing lightly on those locations, you are probably hearing the vibration of the grounding springs on the back panel. Call Samsung (or whoever) and have a technician confirm the problem. The easiest fix is to insulate the offending spring by applying some felt tape.