Dodge magnum vibration
 

I bought my silver 2006 Dodge Magnum R/T RWD a couple months ago. I have since tinted the Windows, added a chrome kit, K&N drop in filter, catless mid-pipes, flowmaster American Thunder mufflers, 3 1/2" x 18" exhaust tips, (the sound is awesome), new struts and shocks, replaced all the entire front-end and rear wiper delete. I noticed something in the rear of the car (inside) started rattling/vibrating when I put the flowmasters on. It got worse when I installed the catless mid-pipes. I have been going nuts trying to find this rattle/vibrating. It at its peak right before it shifts. The spare tire and tools are tight, nothing loose in the spare tire well or tray above it. It's a loud vibration and even my wife says "that would drive me insane".

Have any of you had this problem? If so, how did you solve it?

Can you have someone ride back there and try to pinpoint it?

Tried that, had my 12 year old son sit back there while I rode around our small neighborhood. Where he could hear it, he couldn't pinpoint the location. I'm begging to wonder if it's not something inside the rear quarters behind the panels. He pushed on every panel without any change. He said it sounds like it's the whole back of the car.
Very frustrating for sure.