Originally Posted by CampDestinations.com
It was the metal bit INSIDE the rubber valve stem that shook loose.
I was able to use some small needle-nose pliers to spin it back in. I've got a tool for that purpose on it's way to me now so I can make sure it's really seated well. But I am considering the metal stems as I'll probably be changing out tires soon.
I now understand. It was the screw-in metal valve that backed out of the rubber stem. Some guy in the shop that mounted the tire must have been in a hurry. I keep a two pronged valve seating tool in my toolbox. It looks a lot like a metal valve stem cap. You can screw it onto an old rubber stem to make it easier to turn (and to find).