I think I may have figured out the noise, but I'll need to use my sawzall to test out my theory. I am thinking that when I cut the exhaust pipe to fit the new cat and headers; the pipes were too close together. I had to really maneauver them to get them to go together so I could use the clamp/wrap. I believe the two pipes are hitting each other causing the noise. So, we'll see!
Yeah that wasn't it. After I cut the pipe, and re-installed the clamp/wrap it created an exhaust leak. Sounded horrendous. Got some exhaust wrap with the aluminum; that didn't work. Then got Tiger Patch and that worked fine. Noise is definitely still there. I think it has to be the exhaust beating and banging around underneath. I checked all nuts and bolts under the car. Word to the wise... check your sway bar bolts. One of my front passenger side bolts on my ZOK bars was very loose. Scary considering I tracked and hit the twisties this summer. Need to make a mental note to check those every now and then.
I think there just isn't enough space between the HF cat and the floorboard, as well as the pipe and the backbone brace. Surely I'm not the first person to run into this problem... Car runs great, minus the normal 3k RPM hiccups. Need to clean the throttle body and MAF sensor soon.