Buzzing sound from the LED backlight board
My 27 inch 2011 iMac started to buzz in the top left corner (when facing the screen). It was silent for the first 5 years I had it.
Then the buzz started. The buzz sound increases as I raise the brightness.
I changed the PSU. The buzz came back after 2 days. Then, I changed the LED backlight board and the buzz came back after an hour...
There is nothing left to change! If someone finds a fix, please let me know.