I'm so glad that I'm not the only person with this crackling noise problem. I've actually been worried that my wife's ex-husband had somehow planted some kind of spyware on our SP4's and that the crackle noise was from the internal microphone being turned on and off. I'm relieved to now know that it's not that. :)
I've had my SP4 since December of 2015 and it never made the crackling noise until just a few months ago. My wife also has a SP4 that is only about 1.5 years old and it too started making the crackling noise at around the same time mine did.
Both of our SP4's make the crackling noise ONLY when they are NOT plug into the power supply and AFTER they apparently go to sleep.
Hopefully Microsoft can get this figured out soon.