That is normal but I don't like it either. It might be capacitors or something like that discharging. I hated it on the old iMacs with the kettle plugs because the spark was huge and right near your hand. The sooner we get near-field inductive charging on everything the better.
You'd just have a plastic square on the wall and a flat, plastic plug and there would be a magnet on the wall and metal inside the plug and it would just stick on. This obviously means the large square part of it has to sit on the floor but it's not a big problem and it means you have a far greater limit on how many items you can plug into one socket.