That is the location sound from »Find my«-by Apple. Do you share the location of them with somebody? open »Find my« and check the status of your Airpods.
I do not share the location with anybody, but I as the organizer have 2 other people in Family Sharing in iCloud+. I do share some AirTags with some family members, but not AirPods. The sound happens when I have devices nearby.
I just found out that a while back I tried to play a sound on the case to find them, but it did not seem to work. I just opened Find My and saw the attached picture. Thanks for pointing out it was find my! I think it may make a sound every few hours just so they can be noticed since the pinging sound is pending.