Loud knock at idle!!

[Phorbes]

I have my 71 c10 with 454 swap in it. Today I was making a dump run and when I pulled into the bay to dump my trash I noticed a knock that just got louder and louder.

Metal on metal, quite loud and irregular. Like something is floating around bagging side to side.

I pulled out of the dump and drove her home and noticed that there’s no knock or any irregular sounds when she’s under load. Just while idling...

Any ideas what this could be???

Thanks guys
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