Slight crackling/popping sound in wall switch
Usually a bad internal contact. I've had some old switches do it, but even seen new decora style switches do it, when clicked on slowly. Probably advisable to change it if it's old and does it often.
I had one that did it very badly in my basement, it actually scared me enough for me to change it right there and then, no questions asked. I could see sparks almost flying out of it and it would stay in that "zapping" position if I did not push it off hard enough.