Muffler covered under emission warranty?

[63 Chevy Stepside]

I have a rattle coming from what appears to be inside my muffler on my 04 Av. with 48k miles. I had a retired mechanic tell me this is common on our chevy's and that he had his replaced under the emmsions warranty, but he had to fight to get it. I looked up and read the fed emmsions info, but did not see anything about mufflers. Anyone here have input on this issue?

Thanks, Rick