What is that sound when killing the engine?

With all do respect Vince, Mine was the complete opposite. When my rpm's were high I couldn't notice any noise or vibration's. Only when it was a steady idle 1000 rpm that's when I heard the "box of rocks" sound. Now if everyone equates that sound to only one specific problem sure I guess there right, but seems to me the "box of rocks" sound could be other problem's as well.

When driving at high rpm there was no rattle due to the piece of core that stayed put from the high exhaust pressure, Or I just couldn't hear it because of the engine. But when at idle it was rattling around like a box of rock's. When you see the pic below you'll know what I'm talking about. This is just my experience and Mike I'm not a mechanic and the boy's on here know a hell of a lot more them me that's forsure. This was just my experience.

Hope you get this sorted quick and it's nothing serious Mike.

Cheer's
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It could very well be the cats. Mine has had the exhaust shut off rattle for awhile, but today I drove it to work and noticed some weirdness with idle sometimes and a rattle when I got home. Jacked it up, took off the cats and one had broken up media in it. For a quick fix I just gutted them out and stuck them back on. Sounds good and fixed the problem, so I'll probably just leave them that way since we don't do emissions tests here in SWVA.