I was changing the belts as it had started to squeak. Rebuilt and back together but I notice the main tensioner seems bent over. Its pulling the new belt off and into the tensioner bracket slightly. Could have been like this for a while. Checked the idler and the bolt. Both seem fine. Installed it back to front just to see if it made any difference but it still runs off to the back. So assuming whole new part. You can see the belt has run to the back of the idler in the picture.