The lamp ballast boar of your LG projection Tv is faulty. There is coils, which generate high voltage for the lamp to work. If any one turn of winding inside that inverter transformer gets shorted, it will apply a heavy load to the main power regulator board, and a "Hissing" sound will be prodced. Just replace the ballast board.
This is the picture of the lamp ballast board of your tV. Just look it for anu bulged capcitors. You can see the inverter transofrmer also, and near to it is an electrolytic capacitor. Just look it for any bulging on its top side. I hereby attaches an other picture too, not related with your tV, but for your information, about what a bulged capacitor will look like.
See the red arrow marked capacitors. They are bulged ones. You can see a good one at the top right part of the board with flat top surface. If the capacitor in the inverter board has damaged, the fault you indicates will occur. So check it before replacing the ballast board. OK.