We've asked Apple for comment and will keep you updated. Apple support reps are already collecting diagnostic info, so they're at least investigating the reports.

It's difficult to pin down a cause at this stage. Although the differing levels of the problem suggest the crackling could be a hardware issue, this comes mere weeks after Apple fixed a software flaw that produced crackles on the iPhone 8 and 8 Plus. If it's a related issue, the company could theoretically push out a patch that addresses the problem without replacements. Either way, this and other problems are a reminder that cutting-edge phones can have their share of early glitches -- it can take time before manufacturers iron out the kinks.