Driver door knocking type rattle

Really annoying. It's an intermittent knock or rattle somewhere around driver door hinge area. It sounds metallic, and not like door trim. At first I thought I had left a tool or part somewhere, but no. It happens when the truck moves over certain bumps or pavement in the right way to do it. I'm pretty certain it's not window/lock mechanism, because it's happening farther forward. Checked all bolts. I CAN duplicate the noise by opening the door and pulling up and down on the door to rock it up and down. Door itself isn't loose. Wow. Anybody have something like this? 2000 Prerunner 2WD V6