I had a flowmaster installed and hated it. Like you said, the noise on the highway inside the car is terrible. A resonator won't make a big difference. I ended up switching out the Flowmaster for a Magnaflow muffler. I also had the stock tips welded back on. It sounds great inside and out the looks stock. If you go to youtube and enter Crossfire and 12158, you can see and hear my Xfire.