I have a Craftsman 46" riding lawn tractor 4 years old that has a terrible deck vibration, that I would like suggestions to fix the problem. I have mulching blades that I sharpen every year and balance them. There is no play in any of the pulleys or the bearings, mandrel or shaft inside the mandrel. The idler arm does have some play in it but if it didn't the blades wouldn't be able to engage. I can't find any cracks in the mandrels or any of the pulley assemblies that are welded to the deck. The belt looks ok but it is 4 years old. Could the belt be the problem or the blades even though they are balanced? Any help would be appreciated.