I was plowing today and heard a metal pop in the back. Now the PTO still spins with the clutch out, but the tractor wont move an inch. The left tire barely "pulses" as though something in the drive train is trying to engage. This "pulse" is slow in 1st gear and fast in 3rd gear with no movement of the tractor. There are no bad noises at all. It still runs smooth and quiet. What did I break? Can I fix it? I bought this 9N in the spring, it has sat outside for many years without being run. Got it running great. Plowed 1 small plot no problems. 5 days later I break the tractor on the next little plot.