Have the hood open and watch the engine movement while someone else shuts it off. Engine moves against motor mounts. Could be one or more of the rubber bushings in the mounts that dampen the noise and vibration are squished a bit more so it rattles at tad at various times. An engine torque dampener would alleviate it if they made such a thing for the Xfire... Generally they make torque dampeners for 4 cylinder engines... SRT-4 and other compacts...