didn't know cats can cause so much vibration.... it probably isn't idling properly... which could lead to the cat i guess... does it rattle at open throttle? clogged cats will rattle.... like mine... for all of you with seat issues... if it's loose... just take a look under there while someone sits and reclines and stuff.. if you see something is loose, tighten it if you can or .. I had to put in a little washer to make it tight again... very often it's an easy fix... if it's not, chances are, it's not worth the effort

 

Before, I got a CEL for cat convert problem, but I erase it, and didn't back on again, and it's on bank one, which is the drivers seat side.

So, pretty much, I guess the cat is bad, or not as good as it should be, and thus, the flow does not goes as smoothly as should, and that's what's causing the car to have a bit of vibrate on the driver's seat side.