sounds heard when airpods are in the case

There is a chime-like sound when I put the AirPod case into my bag (it suddenly started); the AirPods are in the case, it is fully charged. It does not stop when taking out of the bag nor when I open case. It only stops when I remove both pods from the case. I am not sure, it may have started after I added standard earphones into the bag. These are switched off, therefore there should not be any ‚communication‘.