Weird issue

I have the  dock for my iPhone 7 plus...usually have a  silicone case on it. Anyways had the phone out of the case last night and plugged it in. It felt as if the back of the phone was vibrating when I slid my fingers across it. Tried a different wall adapter and cable. Still had that feeling. So I tried the original charger and wall adapter, without the dock, no vibrating feeling. Any ideas? Thinking somehow the dock is bad and causing some sort of electric charge on the case of the phone?