V6 Exhaust Question

Currently, I have this kit on my car. http://www.ebay.com/itm/4-5-DUAL-MU...A-ACCORD-V6-/401235477687?hash=item5d6b7f84b7

There is a problem. When the silencers are not in, there is a terrible vibration around 60mph. Even with the windows down, its almost unbearable for some periods of time. But when I put the silencers in, there is no vibration. I dont know if I'm referring to the vibration as a drone or if its the same. If I have change or an empty soda can in the cupholder, it will vibrate it. I have changed the rubber exhaust mounts, I have extended the pipe to where the mufflers come out from under the car a little bit more.

Is the exhaust piping to big? and the exhaust gases are bouncing and not flowing how it should in the pipe?

Or should I go back to the original pipe and weld the mufflers on the end of the original?

I love the sound of the exhaust when the silencers are not in, but i cant stand the vibration/drone.

Thanks

The diameter plays a role, I see it says 2.5" but not sure whether that is the inner or outer diameter of the piping. I would say it's more that it's just a cheap exhaust and it's not tuned.

Ok thanks. I think I'll put my original back on with the mufflers