I had a similar crackly/electrically noise to this but was also a some other noise almost like a beeping in a 2011 MBP late last year.
Thought the noise was from the HD, it was getting old and slow.
Replaced the HD with a SSD, noise seemed to stop but I wasn't using this laptop much after this was done.
At a later date, I was sitting next to the main user of this laptop while it was being used and heard this noise again.
After a bit of investigating, I tracked it down the fan that was making the noise, I replaced the fan and it's been quiet ever since.