Driving over speed bump

Why whenever I drive over a speed bump, I will heard a "gong" sound after the rear wheels exit the bump? I know that the rear suspension is absorbing the up/down motion. But the sound is like some kind of metal being hit by the ground. So I try to use torchlight to see the underbody, but I didn't find any dent on all the parts

I already try to go slowly but still got the sound.... Unless I'm really slowly like <5km/h
Abyone can advise on this?

Wind down the windows and hear? If still have better to send back to C&C for a check, my guess is that it is coming from the springs?