https://techcrunch.com/2017/10/03/apple-releases-ios-11-0-2-with-a-fix-for-crackling-audio-on-iphone-8/

If you just bought an iPhone 8 or iPhone 8 Plus, you may have encountered a rather annoying issue. Some new iPhone users could hear crackling sound during calls. Apple just released iOS 11.0.2, which includes a handful of bug fixes, including a fix for this bug.

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.