You need to replace the rubber idler tire on the idler arm ass'y. You may have to take it apart and measure its thickness and diameter to match up to a new one. Not many parts sellers list idler tires by brand and model any more.
Here's a seller that does list rubber parts kits for your model.
http://www.studiosoundelectronics.com/mbk-07.htm
No affiliation. I do not know the quality of these parts.