Knocking sound in Grand Prix

I have a 1994 Grand prix (180k miles). Whenever the engine is started, the car makes a knocking noise that sounds like it is coming from the top middle area in the engine, on the passenger side. When the engine warms up a little it seems to go away, but when I push on the accelerator it comes back. I did notice the tensioner (for the serpentine) arm moved up and down very quickly when the vehicle was on. I've never seen one move in a vehicle.

Do I just have a bad tensioner that's hitting against something, or should I check for something else?