I had this same problem. Litlle tinny sounding rattle under the dash, sounded like it was coming from the drivers AC vent. Pulled off the under dash cover and found there is a little sheet metal L shaped bracket with a eletronic module mounted on it. This bracket is only mounted from the front dash frame with 2 screws, and extends back maybe 5 inches. Tapped it a few times and got the same sound, just took a couple of tie-wraps looped it through a hole and tensioned it forward...to the steering colum I think...that was 3 months ago and haven't heard it since a really chessy setup.