really? they are new, lol.... guess it can happen tho, just my luck if they are

 

The material came apart in my cats last year. I first thought I had a trans problem because I could hear a rattle come from what I thought was the trans.

Stopped by a shop, the guy got under the truck, hit the y-pipe with his fist hard a few times and they rattled away.

If it is the cats, get them fixed soon as the situation could lead to a big problem, no power, engine lights and believe me I know.

EDIT: I have 3.73's in my truck but it didn't come with the tow package. It's there now though.

 

does it sound like there rattling?
also if the y was made from thin metal you'll hear more engine noise.same goes with headers,some complain about the valve train noise they hear after install.this could be what your hearing tho

 

sounds like knocking inside the cats... heres a cheesy vid clip...
http://videos.streetfire.net/video/k...und_187345.htm

 

I have a 2005 ECSB Z71 with the L33, towing package, and freakin' 3.42's. Somebody needs to be smacked at GM for a sick joke like that.