I have a 2001 silverado 5.3 and when i rev it up sitting still it has a slight rattle at around 1800-2500 rpm it sounds like its coming from right under my feet where the left cat is.
a few people in town have less miles than me and they say they have already had to change theirs at around 170,xxx miles, and mine has 207,xxx. It runs good at higher rpm and doesn't have the low end power it had just a few weeks ago everyone around here tells me its a cat. just wanting a second opinion.

 

sounds like theres a good chance the cat is breaking up.

any chance you can get a video so we can actually hear the noise?

do you have any type of forced induction on your truck? because that can cause the cat to deteriorate faster due to heat and pressure.

 

We had one recently around here that was misfiring, and would barely idle. Turns out one cat came apart and was almost completely plugging the passanger side exhaust. When you shut it off, you could actually hear the sound of pressurized air pushing it's way back through the heads and out through the intake.

There was a rattle coming from that side prior according to the owner. Chances are, one of your cats are roached.

 

Thats what sounds like post a vid that will help

 

Crawl under and bang on the pipe with your fist, next to each cat. Which ever one rattles is your bad one.

 

I just had that same noise. I replaced the y-pipe with an off road one by Jegs. No cats. When I looked at my old cats I could see the driver side one had broken up and im guessing made its way through the exhaust, maybe theres some in the muffler. Got 45 a peice from the local scrapper for them.

 

both of them rattle and when i hit it with a hammer the truck started running better and idled smoother and also my cylinder 6 misfire code went away.
the passenger rattled more than the driver side

 

Cut them open and gut them= Problem solved

 

that's what I want to do but I don't have a welder...
i think I will go down to my exhaust guy and have him cut them off and i will gut them and have him put them back on. he will cut them off and put them back on but he wont (gut) them i have to walk out of the shop and use his tools to do it lol