Sounds like the timing belt tensioner to me, too.
If that's the cause, you're damned lucky the timing belt hasn't snapped.

Remove the rest of the plastic cover on the far right side of the engine and visually inspect the tensioner. I bet that it has failed, causing the belt to go slack.

If it is the tensioner, then I would replace it, the timing belt and the water pump. You cannot afford to have the timing belt fail while the motor is running...because pistons and valves will collide if it snaps.