The upcoming iPhone X and a lack of major upgrades over its predecessor saw turnout for the iPhone 8 at its lowest point in years. Now, Apple has confirmed that some owners are experiencing a crackling or static sound that affects the handsets’ audio, but it is working on a fix for the problem.

In a statement to The Verge, Apple said the issue is limited to a “small number of users” and that an upcoming software release will correct it, though there’s no timeline for when it might arrive.

The crackling only occurs during phone and Facetime calls, though not on every occasion. It’s been reported on both the iPhone 8 and iPhone 8 Plus, and across different carriers in multiple worldwide locations. The randomness and the fact it doesn’t appear when using the earpiece as a loudspeaker suggests the noise is a software problem rather than the hardware one.

Apple support forum user SOBADA describes the noise as “intermittent - on some calls you can perfectly notice it on others no problem. I have already tried resetting the phone with no success on solving the problem. I’m dealing with the issue since Friday. Very disappointed! Does anyone has the same problem or knows how to fix it?”

Many locations that have famously seen huge queues during previous iPhone launches were noticeably quiet during last week's iPhone 8 release, including San Francisco’s Union Square and Apple’s Sydney city stores.

Apple isn’t the only company to experience audio problems in its newly released handsets. Not long after their launch last year, a number of users found the sound in Google’s Pixel phones distorted when the volume was at its highest level. A problem the company blamed on the devices' hardware.