1998 300 honda 4x4

Look in the repair and maintenance section of the forum the first stick by Saber6 is what your looking for.

Timing chain guide is worn possibly causing slack in the chain

Im not real sure but my quad does the same thing as it gets hotter the oil thins or gets looserwhen you have an ohv engine with no lifters you have basically a solid valve train in a sesns so you may hear a rattle and more defined as it get hotter.
Also does you quad have a back-up kickstart? if so the detent lever may be the culprit.

NO No No NO You never adjust valves on a warm engine.... You always do it on a cold engine let it sit overnight then make your adjustments.