I have a 2005 dodge neon sxt. i found a full exhaust of a srt 4 (cat all the way back). I had it installed but over the months everthing is starting to make noise, its like the noise when you have a system installed. The front dash raddles where the pilliars connect (front left corner) front right corner) plastic pilliars connect to dash. Inside the door panels. the mechanic says its from the Frequency from the 2 resonators in the middle of car, The noise seems to be most presant when taking off and coming to a stop. i have had all motors mounts replaced i have checked everything i can think of but its driving me nuts enough where im tempted to put stock muffler system back on, but i love the srt 4 exhaust on it. (just not the maddening part) I am willing to try anything..

 

It could be that at lower RPMs the exhaust could be reaching the frame's resonant frequency. That happened in my previous Chevrolet, the dash would rattle at a higher speed.