I have a 1990 Kawasaki 4X4 Bayou 300 cc. The engine runs fine
except when engine is under no load (just driving or letting off the gas when slowing down).

Then it has a knock in it. It did not do this until now. I had it at two different dealers, and they have no idea what it is, and they are not that interested in tearing it down to find out. It has not been used a lot and should be in great shape. Is this going to blow up? Any idea what this could be? This is not normal, is it? The dealers don't know.
Thanks
Land King