Anyone who uses hearing aids can experience occasional whistling or squealing, which is known as acoustic feedback. Since the components of small hearing aids are so close together, amplified sound can leak back out of your ear and get picked up by the hearing aid microphone, creating feedback. The easiest ways to avoid feedback are to be sure the hearing aids are fully inserted into the ear canal and to be sure the ear canal and the sound opening of the hearing aid are free of earwax.
Today’s sophisticated hearing instruments are very good at controlling acoustic feedback, so it is not as much of a problem as it was in the past. If your neighbor’s hearing aids are whistling all the time, he might be using older hearing aids or his hearing aids might need to be checked by an audiologist.