Its a manufacturing defect. When an automaker produces a car it does tests on the differential (where the drive train gears connect to spin the tires) in order to make sure their is no "gear whine". Gear whine occurs when these gears are slightly offset in production and over time wear away at each other (very slowly, too slow in fact to disable you from driving in the normal life of a car). When they wear away they make a high pitched humming noise usually which changes depending on acceleration or deceleration. Obviously the XFire is rear wheel drive, however usually it is trucks that have differential problems not sports cars. A way to check if your differential is off is having the dealer or mechanic take off the differential cover and check for metal shavings (almost like a powder that you might find in paint) around the differential. As you can see from this post and the link benz47 posted earlier of another post on the same topic, some '04 XFires were built with this defect. However it is was rare enough that Chrysler did not create a TSB on the issue, therefore depending on your dealer (especially clueless ones) its harder to have them fix it or even know what a differential gear whine sounds like. The thing about the problem is, although the differential was built slightly off kilter it can take thousands and thousands of miles of driving for a hum to actually form. My car has 49,000 miles and I just got it used. I don't know how long it has been making that noise but that many miles just goes to show you how long it can go unnoticed. Also, if you have gear whine in the differential it only gets worse with time. Meaning those gears keep grinding against each other and the noise just gets worse. This is how much I have learned just doing research on the forum and talking to several mechanics I think its correct basically. I hope this helps!