Cults are among those groups that sometimes use sound and noise transmissions to frighten and harass. There are also devices for making lights that are turned off glow momentarily.
Also big are pinhole spy cameras, nanny recorders and the like. Peeping Toms and other perverts have been caught with those.
To discover more check into the Watchtower Security companies in and near Santa Rosa or look at websites like
http://www.pinholespy.com/
http://www.spycameras.com/
http://www.advanced-intelligence.com/video.html
http://www.kgbcameras.co.uk/cameras/spy-camera.htm
The voice and light devices sold by companies like those above pale in comparison with those used by the CIA, military, cult and paramilitary organizations. Theirs are smaller, sometimes microscopic or close to it.
Cultists who send pedophiles among those going door to door are scared stiff of being discovered or having it pointed out what they are doing. They are weakling and YOU, not they, are actually in control and have greater power.
Some interesting sound effects are that they may play noises of the wind howling, crying children or crowing roosters, a cat meowing, a cricket chirping, the voice of your marriage mate calling out to you and the like.
Another electronic cult tactic is to place a micro-transmitter in your bedding then play low-level words or phrases while you are asleep for example: "God, storm, fear, return, return." This could have the effect of your waking up, thinking something like "I dreamed God wants me to go back to the kingdom hall."
Lots of civilian contractors work for the U.S. military which helps them out with its expertise. Sometimes the contractors retire, join a religious group and, still having some of the best devices in the world, use those to help the religious group. Also, the cults sometimes have their own security companies or have close personal or business ties with security device companies owned by fellow cultists.
That's my take, anyway. Time to time others have complained of the same stuff going on on this same jehovahs-witness.com website, and I believe this is the logical answer for most if not all of it.