Yep, that's probably because the power adapter wasn't grounded. Here in Europe, at least in 2013, the adapter had two ways of plugging into an outlet: a cable

Yep, that's probably because the power adapter wasn't grounded. Here in Europe, at least in 2013, the adapter had two ways of plugging into an outlet: a cable and a plug that could be clipped on. The plug only had two prongs, while the cable had a ground connector. I'd get the "buzz" if the adapter wasn't grounded. This also happens with my HP laptop if I plug it into an ungrounded usb-c adapter, so I'd expect the same thing to happen to iPads.