Is the hum comming from the switch of or the light? Is the light in a dim status? I have noticed that lights dimmed using an X-10 wall switch tend to make a horrible noise, altough lights controlled a plug in module are quiet. The key is to make sure the lights on the wall switch are full on or off. The second you start dimming a wall switch the horrible sound starts, and it gets worse the more you dim it. I have several lights using plug in modules and wall switches, everytime I dim them the wall switch controlled lamps create a horrible noise. I have several wall switches installed and we started hearing a bad hum, after my In-Laws came to visit, a couple of days latter it was pitch black and we noticed a subtle glow from certain fixtures. The In-Laws never turned them off, just dimmed them to point in which they were not visible. We hit the few switches, and stopped the noise.