I have a rattle under my car, can't find an obvious/loose heatshield. I understand the cat can rattle - I have 79920 miles (so emission warranty is about to go). How do I check the cat (hit it with a wood block and listen for rattle)?

Hate to have a bad cat and barely miss the warranty.
Dealer cannot get me in before Monday, and I will be past 80kmi by then . . .

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)