Been there. When we were at the Rally in Dearborn, the GT engineers (not techs) took my car for a ride to see if they could find it. No washboard streets, so no rattle. Finally, I found it myself. It's the overhang over the tach, kind of a hood to prevent glare. Put your hand on top of the dash and hook your thumb under the overhang. Kind of squeeze the two together and the rattle will go away. It's the plastic in the overhang vibrating against the top portion of the dash. I've pushed it several times and haven't heard it for many months, but haven't driven the car much.