Take a recording of the hum of your desktop computer by setting an iPhone next to (ideally, on a towel or pillow) your computer and using Audio Memo to capture

Take a recording of the hum of your desktop computer by setting an iPhone next to (ideally, on a towel or pillow) your computer and using Audio Memo to capture a clean recording of the hum. Then open an AppleCare support case and indicate that your AirPods stopped noise canceling a certain hum in your house a few weeks/months ago, and that you'd captured a recording of that hum with an iPhone and would like Apple to evaluate whether your AirPods are defective or the hum is by design found not to be addressed by their noise canceling. At the very least, they'll probably let you provide them the recording (Support has a way to capture files electronically), and at best, they'll either execute a capture/swap of your AirPods for testing or tell you that this is within expected tolerances but thank you for the recording in case it helps them improve in the future.