I know it's been a while since this post, maybe someone is still following that has an answer.
I just inherited a iMac 27 Late 2013, i5 3.2, OS X 10.12.2. I've tried all the suggestions I could find.
Grounding, Chrome Flash, Faraday Cage, disconnect all peripherals.
When I started using the machine with a clean install, there was the wonderful silence, but over the past few days I got the snap crackle pop, static sound. It literally varies minute to minute. It is slightly effected by screen brightness, but just slightly.
I would so appreciate any leads.
I can't return the machine.
Regards,
G