Yea they use a modulated signal/voltage. The advantage it doing that is you can carry higher current over a smaller wire. AC will carry further over smaller wire than DC. They just picked a Frequency close to/in the audio spectrum. So yo are hearing that signal or a harmonic of it. Not that unusual. Wish I still had access to a sillyscope I'd love to see what it looks like.
Damn I envy you folks that can hear that good. LOL