Time for random speculation. I guess it was the hazard sign mounted on the back of the tractor, mine clattered and rattled until I bought ear plugs.So....
Your not going to say what it was???
At that time it was a loose fan belt that was slapping the guard. Since then I believe the new clatter is valve clearance needing adjustment. No adjustment has ever been made so I'll see if I can get to top of engine and clutch housing for timing mark. I have the shop manual so it shouldn't be that difficult.