Hi so I’ve been having this exact same issue. Personally the only way I can explain the sound he is hearing is it sounds almost like the Mario power up sound. That’s the best I can do for you.
also replicating the sound. I have found success in replicating the “Mario power up” sound by removing both AirPods from the case and waiting for the case light to disappear. Then place them both in the case and close the case and open the case very slowly. But do not let the light go on. You are able to open the case very slightly without the case light switching on. Anyways then hold the case near to your ear and listen for about 30-45 secs and you will hear it. Sometimes each AirPods does it individually or sometimes they both do it in sync.
also I’m using AirPods Pro 2nd Gen with USBC(A3048)