Failing bearings will often make a noise when they start to fail. The noise is not always easy to identify. The use of a stethoscope can make pinpointing the problem bearing easier. Once the vehicle is both on the rack and in gear, you can use the stethoscope. Start by placing the stethoscope on the knuckle as close to the bearing as possible. If the bearing is failing, it will produce a noticeable hum or grinding sound. It may be necessary to turn the wheel slightly to duplicate the bearing noise on the rack.
Caution: It is essential to take extreme care when working near a rotating wheel.