Originally Posted by chuck65
A good way to detect bad bearings that's quite simple is to use a large screwdriver and put the blade on the alternator, while the engine is running, and put your ear to the handle. If the bearings are bad, the noise telegraphs up the handle and you can hear them rattling around. We used to use this method in the garage. Another way is to loosen the belt and spin the alternator, sometimes you can feel the grating of the bearings. Other causes of the noise could be from other devises driven by the serpentine belt such as the water pump or idler pulley or power steering pump.