It sounds like the rattle I was experiencing....for me it was a hanger and I checked three times and everything seemed tight...someone from the forum posted this and I did exactly that and it fixed the rattle...
It's most likely one of the exhaust hangers from your after market axle backs, I had the same problem and so have dozens of others. One of the front hangers is vibrating against the metal bracket that houses the rubber bushing that your hanger slides into. Just take a pair of vice grips or channel locks and bend the hanger back a little nearest the 90 degree angel bend toward the front of the car. That should take care of it.
2013 Mustang GT/CS: Black On Black/6 Speed MT/BAMA 91R Tune/Glass Roof/Electronics Pkg/Comfort Pkg/3.73s/Roush Axle-Backs/C&L CAI/JLT Oil Separator/Tint 30/5
Gripping the wheel, his knuckles went white with desire! The wheels of his Mustang exploding on the highway like a slug from a .45. True death: 400 horsepower of maximum performance piercing the night...This is Black Sunshine-White Zombie