So I have a 2013 g37X it has a muffler delete and just recently I noticed a rattling noise when revving and driving. Does anyone know what this is??

 

Possibly heat shield on cats

 

Is that a quick fix?

 

Search and find out.

 

Yes if you have access to the the bottom of your car via a lift or jacks.

You have two options if there is enough meat left on the shield you can use a hose clamp to hold in place.
If there is not enough meat to use the hose clamp to hold them on just pull them off. (or buy replacements)

Again very easy if you have lift. (basic mechanical skills will do)

 

Take a video and post it. I have bad cats, the sections inside have broken down and started to make noises on my car. If its particularly bad at 1600~2000 rpm while the motor is slowing down, I'd bet its the cats.