Found the same issue on reddit:
Anytime a sound transmission ends the headphones emit a loud and deep "BONG" noise each time. Can be anything: pausing a song, a ping from Microsoft TEAMS, a warning sound from Windows. It makes keeping the headphones on unbearable. Been going on for at least 3 months.
BOSE 700 NC
Windows 10 on Lenovo laptop
Android on Samsung Phone
Restarted headphones, un-paired and reconnected to phone and laptop, Updated firmware