If you've been hearing a chiming sound from your AirPods Pro 2 case when the AirPods are charging, it's a feature that Apple added with the launch of Hearing Health last year.
In a support guide, Apple says that the AirPods Pro may play a sound every so often while in the case to ensure the microphones and speakers are working as intended. From Apple:
To help ensure that your AirPods microphones and speakers are operating at their best (for example, to help provide high-quality hearing test results), your AirPods may periodically play a quiet chime when they're in their charging case.
Information on the mysterious chime was highlighted on Mastodon after Apple's unclear AirPods sounds were discussed on today's ATP podcast.