thats sound u heard is the sound of the plastic bumper flexing, also adding to that sound is the foam bar reinforcment and metal reinforcment doing its job and contracting. what u need to do is check all connecting points for cracking, check if any clips are missing or have been losened, even though theres no external damage the metal reinforcment connecting points (top and bottom bolts) will be bent, how much will depend on how hard he's hit your car. if you can, remove your bar and have a look, u might find that its not even worth making a claim but its up to you