Q: when placing gear shifter in reverse I hear a pop/knock.

Hello. A popping / clunking noise when moving in reverse can be caused by many things. Two of the most common causes for those symptoms are worn axles or motor mounts. The axles are the joints that transmit power from the transmission to the wheels. Most axles have a flexible CV joint that allows for a range of movement for the wheel. This joint can wear out over time and cause the symptoms you are experiencing, along with clicking during turns. The same can also be caused by worn engine or transmission mounts. If any mounts are worn they may allow excessive movement of the engine or transmission, which may result in noises when the transmission is put into gear or when during acceleration. The same can also be caused by internal transmission issues, though this is less likely if the noises only appear in reverse. As noises can sometimes be difficult to diagnose, simply due to the amount of moving parts on a vehicle, I would recommend having a professional technician, such as one from YourMechanic, come to your location to diagnose what may be causing your noises.