Keyang Accident

[MrForbes]

Two clues: 1. You heard clicking when the arm fell (I assume the motor was trying to hold up the weight of the arm). 2. You hear the same clicking when you run the arm motor to "stall".

It sounds to me like you have partly damaged the spur gear teeth inside the motor, and it can take only part of the load it is designed for, then starts to slip, the clicking sound is the gears skipping teeth as the worm gear turns and the spur gear just stays still.

Sounds like you need a new motor, and probably a few spares as well, or figure out how to make sure the load on the motor is never high enough to damage it again (which is hard to do!)