Crackling Sound from inside

I have the aforementioned iMac and it's had this constant crackling sound coming from behind the screen for a long while now. It's not from the speakers. The sound will become present after the first power up and will remain even when powered down but at which point it becomes a faint ticking sound.

I recently opened up the iMac and changed the LCD inverter board and it did not cure it. GPU also recently replaced but no change to noise. I thought it might be the inverter because if I increase the screen brightness the noise gets louder and on reducing brightness it becomes quieter. Also, if I click on windows or icons or something, a short burst of louder noise is made.

I have run CPU and GPU intensive benchmarks but the noise is constant during these times.

On inspection of the power supply board, nothing seems obviously broken. As the noise is present when the iMac is powered down but plugged into the mains, I guess whatever component it is must be receiving power during this state. Anyone know what it could be?

Sometimes the noise is so bad it seems to be making my room light flicker with it so obviously it's putting noise back into the mains. Is it most likely the power supply or is there some other component that can do this?

Thanks in advance for any help