Clanking noise? Have you determined more specifically where in/on the engine that noise is coming from? Put a stethoscope on it or just opened the bonnet and listened carefully to try to isolate the source of the noise? Does the engine still run okay? Is it developing normal power, or is it low in power? Have you done a visual check to determine if there is anything visibly loose that could be making the noise, such as alternator, water pump, etc? How does the exhaust look - white smoke, grey smoke, black smoke?
I'm asking all these questions because just saying there is a clanking noise is impossibly vague. We have to have something more to go on to begin to determine where to look. It could be a thrown rod (unlikely, as those make a god-awful racket and then the engine dies), a bent pushrod, a broken lifter spring, a loose exhaust manifold, etc, etc, etc.
Some more background info would be helpful, too. Was it properly serviced/set up when you first got it - clutch adjusted, fluids changed, etc? Was the valve lash set at 50 hrs? Head re-torqued? Did it get all the required fluid changes at the suggested intervals? Have you checked the engine oil for water contamination? Checked the radiator coolant for oil contamination? Are the fluid levels all where they should be?
What were you doing with the tractor at the exact time it suddenly began making this "clanking" noise?
If you can supply the answers to most of these questions then someone here can make an educated guess as to where you should investigate further to determine what is going on. Without those answers no one could possibly guess, except by wild-assed luck, unless they have ESP.
