Another Exhaust Question

i'm wordering if anyone else has had this issue. i put a flowermaster 40 on four months ago. i used factory cats and cut old muffler out. Welded and rehung system. now i feel a vibreation and a low viberation noise. you can feel it in the passenger side floor. I have inspected around the front fender well and uca and lca header ect.... The only thing i can come up with is the exhaust. anyone?

 

Since you said under the passenger floor it might be the heat shield for the O2 sensor or the heat shield on the catalytic converter itself. The muffler is a little more rear ward when installed.

 

I did check the heat shield but not were the O2 sensors is located. Thanks for you insite.