Well it sounds like a GM performance catback making sounds like lose metal, yesterday I replaced my DP with a ZZP 2.5 with no cat, and heard some metal sounds coming from the rear "catback"

What can be done, is there a life time warranty on this mullfler.

Any sugestions

 

Had to dig out the warranty on a PDF, here's what GM covers on their performance exhausts.

If installed by a GM dealer before delivery, or during the new vehicle limited warranty period, the exhaust will carry a limited lifetime warranty for the original purchaser only.(Parts & Labor)

If installed by a GM dealer after the new vehicle warranty expires, the exhaust will carry a 12 month or 12,000 mile limited warranty.(Parts & Labor)

If installed by a non-GM facility, or the purchaser, the exhaust will carry a 12,000 mile warranty.(Parts Only)...Purchaser must submit a dated receipt to GMPP.

Before condemning the muffler, check for interference under the vehicle, something might have been tweaked during the downpipe installation. That being said, you may have a loose baffle in the muffler, an easy way to check, give it a good hard thump with your hand.