I have a pretty good idea what it is
This is a very timely thread. I have a 2011 GT, Roush Supercharged, Borla exhaust, and various other mods. My car has developed a very similar issue and has been at my local dealer multiple times. We thought it might be the diff, axles, etc. I have been documenting everything just in case things need to be escalated. I will spare you all the gory details, but I went down and drove the car today after they removed the panhard bar. The noise was GONE. I think there are 2, but this is the one that has 2 bolts on the left side directly near the left exhaust. Evidently things can loosen up over time, especially when mods and more power are added to the car. Tomorrow, they are going to reinstall the bar, torque everything properly, and we'll do another test. The good news is that it's not anything major. Hope this helps.