Mine does it too, every wot shift. I'm thinking that the belt is part of the problem, but I've also seen a number of tensioners that stick just outside of the range of movement where it operates. In other words, if you take a look at the tensioner while the engine is running, you should see it working back and forth in a narrow window of movement. I suspect that when I do get some time to check it, I'm going to find it binding up just outside of its range of operation...I hope. Please keep us posted on your results too!
Mike