Some iPhone 8 owners are running into an issue with the phone’s earpiece: when they make a call, the earpiece makes a disruptive “crackling” or “static” sound that disrupts the audio. Dozens of people on the MacRumors forums, Apple’s support forum, and Reddit have reported experiencing the issue. It was also present on one phone owned by a Verge editor.

Apple said in a statement that the issue only comes up in a “small number” of cases and that the company is working on a software update to fix the problem. “We are aware of the issue which is affecting customers in a small number of cases,” an Apple spokesperson said. “Our team is at work on a fix, which will be included in an upcoming software release.”

It’s not clear what’s causing certain iPhones to be affected. The problem seems to impact both the 8 and 8 Plus, and owners indicate that it’s happening across different carriers and in different parts of the world, too.

The crackling only occurs during some phone and Facetime calls, though — it doesn’t happen during every single call, and it doesn’t show up when using the earpiece as a loudspeaker. That suggests it may be a software problem, like Apple is saying, since the hardware itself isn’t consistently displaying the issue.

You can get a sense for what the crackling sounds like in the video below — you’ll probably need to wear headphones to hear it:

The noise sounds like sharp pops at the very edge of the earpiece. While it wasn’t totally disruptive during the call I heard it on, it did sound a little odd and annoying.

Apple didn’t offer a timeline on when the bug fix would be shipped, but it typically issues multiple updates in the weeks and months after putting out a new phone and operating system.

This is the second software fix Apple has promised for one of the new devices it shipped this month. A software issue with the Apple Watch Series 3 caused the Watch to connect to unauthenticated Wi-Fi networks instead of handing off to LTE, leading to some real problems, in our testing, with the device’s flagship feature. The iPhone 8 issue seems to be less of a problem, but it’s still an annoyance for some of the device’s earliest buyers.

Additional reporting by Chris Welch.