My 23hp vanguard idles smooth and runs out good. Lately at part throttle there has been a really loud rattle. If I run wide open it isn't there. The oil level is good. Is it possible something is floating around inside the muffler? Everything I have checked seems tight.

Buddy of mine has a 23 that does the same thing and has for a few years. No malfunctions yet!

It is usually the factory air box. I added some foam tape inside the top cover which makes it hard to put the cover back on but it does not rattle any more.

One time it was rattling an extra amount and the wingnut holding the air filter on had completely worked itself off and was bouncing around inside the airbox, so I keep loc-tite on it now.