Well let me tell you what I did and you can tell me whats wrong.
I cut the factory y pipe off the car right after the two converters. I installed a short pipe with turndown on the right and little longer pipe with turndown on the left. Essentially the turndowns are right under the seats. It sounds awesome,however, when I rev it up you can hear some serious tin sounding vibration. I checked the pipes and they are not hitting the floor board or making contact with any other metal. Where is this annoying noise coming from???? Need to find it, it's driving me crazy!

 

The pulse from the exahust alone could be rattling the heat sheilds, so nothing will be touching, sux don't it....I think I have the same problem

 

I have the same problem as you are describing. I am used to it now.

 

I have cutouts on my car that are directly under my seats and whenever they are open I have lots of rattles in my car.