Have a Cub Cadet with a Kohler 26 hp courage motor. Bought it in non-running condition. The ignition coils were bad and replaced them. Adjusted the valves and it runs great. Starts right up weather its hot or cold. Does not miss or backfire runs perfect. But when warm at a idle sounds like a rattle I've checked the valve adjustment a few times when cold and they are right on the money. Just trying to figure out the rattle in it?? Sounds like valve train but runs almost perfect and has plenty of power. Could it be the gap on the coils need adjusting ?? Would the the gap on the coils cause detonation at an Idle and not affect the performance ?? Really starting to pull my hair out with this one ??
Could be something as simple as some sheet metals or shields rattling or something with the exhaust. If it sounds normal at other speeds and doesn't get worse I would be thinking that you are over thinking it.
Check the mounting bolts are tight
Take the blower housing off the engine when warm and use a screwdriver as a stethascope to see if you can identify where the rattle is coming from.
Lots of things can make a funny rattle like a bit of gravel caught in the fins right through to a loose big end
