Picture slowly started getting darker, completely out, but still has sound.What could be the problem?

The inverter board, which is part of the power board is slowly dying, so replace the power board.

With a DLP TV, getting a darker picture means it is time to change the bulb.

Contact any service technician. Its CRT base connector socket, or, LOT is faulty. This is not a user repairable fault. Replacement to any one among component stated above, or both, might need replacement. Gopakumar Gopalan Google

the picture getting darker means the lamp has aged and u need to replace it another thing u need ti replace the mirrors in the light tunnel of the light engine they have faded secondly the noise means the colour wheel has aged you need to replace it u will have to replace the whole light engine cos the color wheel comes inbuilt the light engine and u will be ok ontact lg dealer in your area

It sounds that the horizontal output section circuit to your TV is to be checked. It is really a skilled work. Contact any service technician.

REPLACE THE LAMP IT IS LOCATED IN THE BACK OF THE TV THERE ARE INSTRUCTIONS IN THE OWNERS MANUAL FOR REPLACEMENT IT IS CONSIDERED CUSTOMER REPLACEABLE ITEM.
PART NUMBER 915B403001 Lamp (180W)

It sounds like your lamp is about to die out,this tv lamps are good for 6000-8000 life hours witch is a average time of 2-4 years old and with time the picture starts to get darker and darker until one day black screen here is a link with the lamp you need

http://www.discount-merchant.com/SearchResults.asp?Search=BP96-00608A

That would be the first thing to try.

Wow you have a serious problem. Your LCD screen is losing power probably from a leak or bad wires or connections. If you stil have your warrenty, I would try to take it back.

The cartridge itself needs to be replaced, threre is a blade called the drum cleaning blade which scrapes toner off the Drum(blue roller) it is not cleaning the excess toner.