JUST A HEADS UP, IF ANYONE HAVE A 2014 and up impala with less than 100k miles with this same issue you may want to finish reading this post!!! I recently purchased a used 2014 LTZ impala and maybe a day or 2 after i got it home it developed a rattling noise every time its starts up in the am and after about 15 sec it pretty much stops.. but while driving i can tell it has a exhaust leak somewhere bc i can hear it at times... anyway i took it to a muffler shop and they looked at it and they showed me where the leak is and they also stated that they wont mess with it b/c i need to take it to the dealership bc it should be covered under the federal emissions 10yr 100k warranty. The leak is a cut at the manifold where the cat bolts up to the engine and its all one piece. Anyway i took it to the dealership and sure enough they said it was covered but it was an 8yr 100k emission warranty. so they ordered the new parts and i'll go have it installed next week sometime...