Intermittent Light Buzzing noise

Hello!

This is incredible and Frustrating at best! I purchased my iMac 27' (mid 2011) Dec 2011. The light buzzing noise just started yesterday! Last night I unplugged power source and held the Power button for 5- 8 seconds, and when i turned on my iMac- no more noise. Watching a movie just now, the faint buzzing noise is back. I am thinking about replacing he LED backlight and or the PSU. Should I try to purchase online or go through Apple? (no more warranty) - I'm so sad. Previous to this Mac I had a G4 which lasted me 12 years! (believe it or not- I shut down ever night). Any suggestions would be greatly appreciated. -Toronto Canada