Loud Hiss of Death Fixed on my OLED77C8
My New OLED77C8 had the dreaded Loud Hiss of Death! The sound could be heard from across the room and totally ruined the TV. As others expressed, the hiss was content dependent but became louder with black or near black content and also louder the longer the TV was on - becoming completely unwatchable/unlistenable very quickly (internal speakers off, using sound bar or 5.2.4 surround sound system).
Good News is that LG Service came out today and replaced the power supply and motherboard in about 30 minutes and that totally solved the problem. The 77C8" is now as dead silent as my OLED65C8 always is - irrespective of the content.
Understand that changing the power supply and motherboard hasn't helped everyone but wanted to give a status update that it did resolve the issue here and the LG Tech said it was the worst he'd ever heard- so there is some hope.
Do not let anyone tell you the loud hissing noise is normal, you are not crazy, this is a issue with this TV and once it's fixed - it's like day and night.