blazerboy4

I have a 2005 f150 STX 4.6L. I bought the truck just few months.. and no problems yet. I started hearing a rattle, thought it was something in my bed, cleared out my bed and it still persists. I checked all the heat shields, still mounted in place, good clearance from the cats and muffler etc. When I drive with the windows up, it sounds like it's behind me, but with the windows down, it sounds like it's right underneath me. the rattle only exists under load, but lower RPMs (i don't have a tac or i'd give exact numbers). I did notice a hanger detached from the muffler, so i removed it, but it looks like the weld where the hanger was on the muffler is bad, and there is a tiny gap in the weld. Could the outer walls of the muffler be creating this? Really hoping it's not a cat...

side note, only see two cats on my truck.. is this right?

Thanks for you help