https://arstechnica.com/gadgets/2017/10/ios-11-0-2-arrives-to-address-the-crackling-iphone-8-audio-problem/

Apple has made iOS 11.0.2 available on supported iPhones, iPads and iPods. It's the second update since the launch of iOS 11 on September 19. Like 11.0.1, it aims to fix some problems that users have complained about. Among them is the iPhone 8 crackling audio problem. If that's something you've been experiencing, you can download the update now.

iPhone 8 and iPhone 8 Plus owners had taken to forums and social media to report a crackling noise when making phone calls over cellular networks. Apple acknowledged the issue when it came up and promised a fix in "an upcoming software release." Now it's here.