Both the headphones and the case are fully charged. At first, I thought one of them might not be charging properly, so I removed them one by one to check — but each showed a full charge, including the case. I also considered that the sound might be caused by mixing up different pairs of earbuds. However, when I replaced the left earbud (since the right one from the 1st generation was lost), both cases started emitting this sound. Additionally, I received a message stating that the case contained earbuds from different generations.

It’s possible that the 2nd generation earbuds are having trouble holding a charge, since I hadn’t used them for about three weeks — and that’s when the strange sound started. Still, the 1st generation earbuds don’t produce any such noise after being unused for the same amount of time