Rattling Noise On 2011 Altima Coupe

I originally submitted a question on a 'humming' noise a few weeks ago.  Dealership replaced muffler, noise was gone for a day, and is now back.  It is def coming from the exhaust area (or at least it sounds like it is coming from that area)  Dealership is now saying that the noise is normal.  Unfortunately for me, I can only replicate on occasion, so nearly impossible for me to have them listen to it.  Again, it is only when I get on the gas - any stay stable for a few seconds at 3k rpm's for example.  And the noise is not always there when the car is cold - it's prevalent when the car is hot, cold....it is extremely inconsistent.  Question I have is if it could be the type of gas I am putting in.  I purchased with 35k miles on it, and I started to use Reg gas.  Is it possible that not putting premium in can cause this noise?  I know that an easy test will be to fill up with premium and monitor - which I will prob do anyway.  But was just wondering.  thanks –