I've had this noise from day one. I also noticed it when I was test driving 3 other XFires. It's like chasing a ghost...it seems to radiate from the seat belt housing and sounds plastic in nature. After trying every trick I could think of, I brought it to a dealer other than where I bought it. The Tech was awesome and did a "TSB Sweep" nationally. He found one dealing with a squeak in the rear quarter panel (particularly the drivers side). It has to do with assembly process. When the rear quarter panel glass is installed, DC uses an aluminum rivet to attach the glass to the frame, top and bottom (the top rivet is the problem). Over time with body flex, the rivet starts to loosen and then makes the most irritating popping noise particularly going up and down driveways and on take off. The fix is a specially made screw and lock nut that they replace the rivet with. Takes about 30 minutes. I would recommend doing both sides because eventually the passenger side does it too. It's an order part and takes about 4 days to get. It's worth the wait, very quiet when done. Good luck.
TSB #23-001-05.