About two-thirds through the episode "South by Southwest", Gibbs and DiNozzo are on horseback, along with a sheriff and artist heading back to town. They are being tracked by Abby and McGee back in DC. McGee reports that they are being followed by a helicopter. Gibbs asks how far, McGee replies within a mile (this may not be absolutely accurate, but for this slip-up, it's accurate enough). Gibbs tells them to ride for cover, then the helicopter rises up from behind a hill.

They are in the dessert with no other sounds but natural sounds. You would be able to hear the approach of a helicopter from many miles away. But there is no sound until the helicopter appears right in front of them.