I have a 2007 Corolla LE with an automatic transmission. After I drive, when I shift into reverse there's a loud metallic snapping noise under the hood. This doesn't happen if I shift into reverse right after I start the car (before I drive). I brought it to the dealer and the mechanics heard the noise but could only scratch their heads.

Any thoughts?

I would take it back to Toyota and demand that they find the problem and fix it. Tell them that you will need a rental car. They just dont want to deal with it but its new so they have to.