I have noticed the same problem this week, and I'm at 2600 miles. Though I generally here it whenever I am going over a bump that torque the rear of the truck side to side, like pulling out into a road where there is a little rain ditch between the sidewalk and asphalt.
Tonight I heard a louder pop and a slight jerk/vibration from under the bed when I started slowing down for a red-light. Got home and checked under the truck, didn't see anything out of whack and it had rode fine the rest of the way.
Going to take it to my dealer tomorrow.