Pull the wheels and clean the hubs. Every Challenger I've owned gets debris between the wheel and the hub and it makes rubbing/clicking sounds. Most noticeable in reverse or slow speeds during a hard turn, like turning into your driveway. You can hear it sometimes at full speed if you pass by a concrete barrier or something on the roadside. But, all of them did this. My 2015 SP, I read that it was the wheel hub+wheel and I just didn't care much. My '18, I chased down and fixed it. Was the driver's side wheel+hub. I pulled all of them and cleaned them with a high pressure power washer + degreaser. My '20 did the same thing. Pulled just the fronts and repeated the process, fixed it.
