Many users have been reporting crackling noise ever since the iPhone 8 and iPhone 8 Plus came out. A colleague also noticed the same bug with two different phones.

At first, everybody believed it was a hardware issue because not all iPhones were affected. But Apple released the first beta version of iOS 11.1 last week and it fixed the issue with a simple software update. Apple expedited the bug fix and shipped a minor 11.0.2 bug fix update so that users don’t have to wait for iOS 11.1.
Read more at:http://infodrifts.com/apple-releases-ios-11-0-2-fix-crackling-audio-iphone-8/