You shouldn’t have a vibration. Post pics of your exhaust setup. Might be heat shielding that is vibrating.

I would put the car back on the lift and rev to the rpm when it vibrates. In may not work without load on the engine though.

My guess is you that your exhaust is resonating at a frequency that vibrates the heat shielding under the car (there is a lot of it). Your car wasn’t designed to have the exhaust dump under the car. You could extend the y pipe dump will an additional foot or so of piping (near your stock muffler exit) and it would probably resolve the vibrations . But as much as it sucks, I would go back to the standard pipes and remove the y pipe.

You may be losing power with the y pipe open. The ecu controls the exhaust flaps in your PE muffler. The exhaust flaps work with your turbos and create the proper amount of back pressure. Some here with the open valve mod have noticed a lack of low end power when the oem muffler valves are turned off and left open. Not entirely sure how the open flap could decrease power, but that is what I have read here.

On my PE, I removed the suitcase resonator. Made the car about 25% louder. If that isn’t loud enough still, get a tune and catless downpipes.

If you are looking for super loud, it isn’t going to happen. The turbos quiet down the exhaust too much. I’ve never heard a C450 or C43 that I would say is really loud.