https://www.theverge.com/circuitbreaker/2017/10/3/16411980/apple-iphone-8-crackling-earpiece-issue-fixed-ios-update

Apple has just released a small update to iOS 11 that has one key change in it: a fix for a bug that caused a crackling sound to occur during phone calls for some iPhone 8 and 8 Plus users.

It’s still not clear exactly what was causing the issue, but it did seem to be a software problem: even on affected iPhones, the earpiece seemed to work normally in most circumstances. But during calls, the earpiece could occasionally produce a high-pitched static or crackling noise. It was faint, but annoying.