Question for Stillen Exhaust owners

Red370

I didnt really notice it until recently, maybe I always have my music too loud, who knows. But right when I turn the car off, theres a consistent thud I hear, not a metallic thud either, and it sounds like its coming from right around the resonator area, i've never had this happen with any exhaust setup in my life. Now, one would think "hey, its just the metal cooling off after being heated", and thats what I initially thought. It never happened with the stock exhaust on my car once. You can actually hear the thud from inside the cabin, and its even more prevalent when you peek down underneath the car. To describe the sound, it sounds like someone is flicking the pipe with their finger around every 8-10 seconds or so. Not sure if this is normal or not, has anyone else had the same experience? I personally torqued each bolt as tight as possible, so i guarantee theres zero chance of it having a leak or loose bolt.

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2011 Mustang 5.0 | 3.55's | Lethal Performance off road H-Pipe | Magnaflow Muffler Delete | JLT CAI/Bama tuned 417whp/ 404 ft/lbs