According to the notice on the support page, Apple says it has determined that a small percentage of AirPods Pro manufactured before October 2020 may experience sound issues. The faulty Apple AirPods Pro may exhibit one or more of the following behaviors:

Crackling or static sounds that increase in loud environments, with exercise or while talking on the phone Active Noise Cancellation not working as expected, such as a loss of bass sound, or an increase in background sounds, such as street or airplane noise

While the cause of the issue is unknown, Apple or an Apple Authorized Service Provider will service the affected AirPods Pro (left, right or both), free of charge. Additionally, Apple also issued the following AirPods Pro advice if Active Noise Cancellation (ANC) isn’t working as expected: AirPods Pro service program covers affected device for 2 years after the first retail sale of the unit. However, this worldwide Apple program doesn’t extend the standard warranty coverage of the AirPods Pro. Users experiencing issues with AirPods Pro can find an Apple Authorized Service Provider, or make an appointment at an Apple Retail Store or contact Apple Support in their region to verify if they’re eligible for the repair program. AirPods Pro (left, right or both) with a confirmed issue will be replaced. The AirPods Pro case is not affected and will not be replaced.