Annoying rattle.

[Licensed to kill]

My car has an annoying rattle that occurs at slow speed but goes away at highway speed. It's a tinny sounding rattle that sounds like it's coming from under the dash or from the firewall. There really isn;'t anything under the dash or on/around the firewall that I can find that would make such a noise but there it is. I just had the motor/trans out for a rebuild and the did not find anything on the outside of the car or under the floor that would cause it which leads me back to under the dash but there is nothing there but the gas tank. Anyone experience such a rattle and find the source??