Answer to: What components need replacing on the Logic board?check the all-in-one cable.... they are notorious on these!
Answer to: Backlight make buzzing soundfor mine, I determined from other threads that the bundle cable coming out the back of the display was bad. in addition to the hissing noise, my display would go blank intermittently. a bit of work to open the display and replace the cable, but it worked... display is on consistently, no noise. HTH.