Microphony occurs with older video cameras. The mechanics of the video pickup tubes of the camera vibrate in response to loud noises (such as gunshots), which translates into regular fluctuations in the resultant video signal coming out of the camera. The end result is a banding pattern across the resultant image.

Dr. Who-The Caves Of Androzani (Episode 4, 4:08)
Note the regular horizontal bands across the image.