Sounds like to me its the cheap Ford flex pipe, on my Explorer the flex pipe section is a corrugated metal with a metal mesh on the outside, corrugate flex section rattles like crazy. Bad design, not sure if the mustang has the same type of flex section. Found the image below online, not my car.
Are you actually driving the vehicle when this happens? Or is it parked? I find I only have the issue when the car is moving. Essentially limited what you can access so that you aren't taking your eyes off the road looking for a song for too long.
I see a PP+NP with the black PP spoiler. I know it was shown like that before but the build and price had me concerned. Glad to see that and clear things up a bit.