To learn more about black boxes and how to test your boat, check out this link, this is chapter 1 of 11 chapters: black box info

The humming you hear is unrelated to electrical charge... it's just a harmonic hum on your cable, it will change with your speed, the amount of cable you have in the water, and your downrigger ball weight. It's like a big guitar string, it's just vibrating as it gets pulled through the water. Don't worry about it.

-N.