I get rocks in my tires constantly from working in a rock quarry, it's annoying as hell. Just have to pry them out from between the tread blocks. Go fast enough and they come out lol. I'd check the tread for little stones or pebbles, and roll it around off the truck. Check the center cap or any beauty ring/ hub cap for a loose foreign object. Maybe leave all that stuff off and drive it after you checked the tire.