ok so i have a 2011 imac 21.5. every time it is even plugged in, it vibrates if i touch it. and it sometimes shocks me. if i have a usb cable from my iPhone plugged in, and i happen to accidentally have my have on the dock connector, it stings my hand... like it's shocking me. it hurts badly. is this normal, or should i take it in to the apple store?