Possible stock exhaust problem?
 

I had a new stock muffler installed on my car by the dealer (startup rattle tsb) about a month ago.

When I hauled out of the dealership, I heard a loud vibration noise that also kinda sounds like a "higher pitched" exhaust growl. This happened from 3300-4k RPM. I don't hear anything abnormal at highway speeds and under 3k it sounds fine.

I knocked around down there a few times and didn't really see anything out of order. I assumed they just slid the old one off and slide the new one on so I'm kinda thinking it's just the exhaust.

Fast forward a few weeks and a few hundred miles, now it only makes that abnormal sound when the muffler is hot and I've been driving around for a while. Even letting the muffler cool off for a few minutes means I need to drive the car around for a minute or so to get it to do it consistently again.

The exhaust smelled like metallic *** when new, and I know there's some "break-in" involved with them, but I'm just wondering if this has happened to anyone else and if it has gone away, or if theres something that loosens up when the exhaust is hot (baffles?) and its defective.

I would like to take it in soon (warranty expires in 3 months), but I kinda half expect them to take it around the block and say it sounds fine.