This is a common Xfire issue. Its usually nothing more than the brake pads moving about in the free play in the calliper. There is an anti rattle device/spring fitted but these can loose the tension they put on the pads as they get old. But it can be made worse if there is wheel bearing play, as the disk touches the pad and shifts it causing a knock. I had this on all three xfires now and again. Simple wheel bearing adjustment is possible for nil cost if needed, and its very rare for new bearings to be needed. I did notice it go after I fitted new pads and disks on one car.