I have a similar problem and finally asked my wife to drive while I jumped in the back seat and listened closely. I traced the rattle at highway speeds to just under the air duct under the driver seat. If I slip my hand between the air duct and the floor board I can actually feel the tap when the noise occurs. When I got the car home I moved the seat all the way up, put my hand in the same place and then looked to see what could be causing this from underneath the vehicle. Believe it or not, it appears to be the plastic shield that protects various lines running front to back. I've had the car in 3 times for this already and just made my 4th appt. This time I'm telling them what to look for since they obviously had no clue!
