Are you sure its in the front of the engine? Did you have someone turn the car off while you stuck your melon in the engine bay to locate the source? You may find such a noise coming from the rear as the engine winds down and the flywheel rattles a bit.
OOOps, noticed its an auto, so maybe this is not as prevalent. Anyhow, my manual does this from time to time, not too concerned.
Dave M