This happened to me four times in four months. Every time, it appeared one of the microphones was broken, because I would hear a buzzing, rattling, or clicking noise when in transparency or noise-canceling mode. The current set have both been good for a two or three months.

At least Apple has great service. When the Apple store was open I could just walk over there and get the broken AirPod swapped in minutes. After lockdown I needed to call Apple support and get them to mail me one, which would take a few days to show up.