After a little more investigation I found that the fan was touching the rear of the casing when running in reverse. I couldn't loosen the adjusting screws going in from the front side of the projector so I heated the plastic fan with a hair dryer and used a screw driver to pry the fan away from the rear of the casing. Each time that I pried a section I ran the machine and then moved to another area until I found where it was touching. I now can use the belt tensioner and proper belt and still have a smooth running projector.