TTI exhaust too loud

I have TTIs on my Dart with a local muffler shop's 3" X pipe on to the Flowmasters then 2-1/2" to the rear bumper. Not too loud but I do get a harmonic vibration at around 2000 RPM. Took a while to track it down. You would swear it was an engine vibration but take the pipes loose at the collectors and no vibration. Connect the pipes back up and vibration is back. It may be that there is not enough, or maybe too much, movement in the entire system. will sort it out in the spring....