Kohler 21hp loud knocking

[ateelander]

My brother tasked me with fixing his Husqvarna. It knocks crazy loud when running. Not much on YouTube other than checking and tightening the bolts to the frame. It's tight to the frame. The block is not cracked so I'm hoping to get it quiet and back to him. Any suggestions other than a teardown to inspect the inside? I've attached images of the tags for reference.

 

[Douglas Lee]

Are either of the idler pulleys hitting the frame?
Thanks. . .
Doug Lee
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[ateelander]

I'll check. Thank you