A ball joint is a part of a vehicle front suspension. Most cars have one or two ball joints at each front wheel. When a ball joint becomes bad, a vehicle could be unsafe to drive; a ball joint, when badly worn, can disjoin causing the vehicle to lose control unexpectedly. One of the possible indication of a bad ball joint could be a knocking noise coming from the front end, particularly while driving over bumps or making sharp turns. If you suspect your vehicle has a bad ball joint or any other suspension component, have your car inspected as soon as you can, it can be unsafe to drive.