For HERO3 camera users, make sure that you have updated to the latest camera software version. The latest update reduces interference from constant noises.
There are times when certain external electrical interference or audio frequencies will interfere with the audio recording of the camera, and cause a background static. This static should only be appearing in your video if it is being exposed to the interference or frequency.
For example, if in a car, you should not have the static if the car is switched off. Electrical systems vary from car to car, so it's not something that will appear in all vehicles or with other external electrical devices that may be causing the interference. Also, all audio frequencies do not interfere with the audio recording, so you should not hear this interference all the time.