Being that I work in a dealer and get rattle complaints on occasion too, all I can say is that unless you can duplicate the problem so that the tech can also duplicate it and then know if he's fixed it later, it has to be duplicatable or he'll just be guessing. It's easy to guess wrong at rattle sources too unless the noise is easily duplicated.

About the worst thing an owner can do with intermittant hard to duplicate rattle complaints is to just drop the car off and have it written on the RO "rattle in car...." without having anyone at that dealership go with you for a ride because if the tech drives it around the block on a test drive and doesn't hear it, nothing is likely to be done.