While getting off the highway a few days ago, I came to a stop behind a line of traffic waiting for a right turn on red. While slowing moving forward with the flow, it feels like getting hit by another vehicle, like a light tap, sounds like it is coming from the rear of the truck. Not using the go pedal to move forward, just releasing the brake pedal. It also happens while making a U-turn in a parking lot. What is it and should I be concerned??