https://www.cnbc.com/2017/10/03/ios-11-point-0-point-2-fixes-iphone-8-crackling-issue.html

Apple just released iOS 11.0.2, which fixes some issues with the iPhone 8 and iPhone 8 Plus, as well as general bugs found in the first two versions of iOS 11.

The big change includes a fix "for an issue where crackling sounds may occur during calls for a small number of iPhone 8 and iPhone 8 Plus devices," according to documentation Apple provided with the software.