This has only happened when revving at a stand still. Vid shows sound. It sounds like rocks an a tin can. Any idea what's causing it?

360 FE, no cats, long tube headers and turbo mufflers.

 

Sounds good to me. Does it have a heat shield loose? I bet the baffles inside your turbo mufflers are broke loose?

With it shut off and you tap with a rubber mallet on the mufflers or try to shake them, do you hear a rattle?

 

No heat shields loose. I was thinking muffler baffles too, but I tapped it with a mallet and didn't hear anything.

 

Maybe a hanger is loose? Guess you get to go muffler shopping then?

 

Hangers are tight. Sounds like it!