All uninsulated lines show corona. [Corona discharge] just [is] not a big deal until you're dealing with a pretty high voltage. As the voltage goes from a very big positive to a very big negative, the air around it gets ionized... This is the normal mains hum... Water is much, much heavier than air, and it ionizes just as easily. So on a rainy or humid day, the corona is pulsing with water in it. This gives it momentum, so the heavier water particles travel out farther. But they themselves are ionized, which means they can ionize more air than the line could normally reach on its own, and ionized air is conductive. And there's almost always 3 of these lines pretty close together. The sound you're hearing is a million teeny tiny electrostatic discharges from all the charged up water particles interacting with each other with nearby lines or grounded objects. This is actually the worst time to be anywhere near them; the air is supposed to be their insulator, and at that moment it isn't working as well.