Most modern vehicles use a serpentine belt to run the engine accessories such as the power steering, air conditioning and alternator. The belt routes around the pulleys in a serpent-like manner, hence the name. A belt tensioner that consists of a bolt plate that mounts on the engine, a spring-loaded arm and a pulley applies tension to the belt. The pulley itself has an outer ring and an inner bearing. If the pulley is making noise, either it's the result of a bearing failure or the spring is not providing the proper tension.
Start your vehicle, leave the transmission in park, set the parking brake and open the bonnet. Listen to the tensioner noise to note if the sound is intermittent or constant. An intermittent noise might signify the tensioner is not holding proper tension, while a constant noise might signify a bad bearing.
Shut the engine off and shine the flashlight on the ribbed side of the belt. If the belt shows signs of glazing, the belt has most likely been slipping on the tensioner pulley.
Push down on the serpentine belt midway between the two uppermost pulleys. If the belt deflects more than one inch, the tensioner is not holding proper tension and you should replace it.
Connect the serpentine belt tool to the tensioner. Rotate the tensioner counterclockwise just far enough to slip the serpentine belt off the pulley closest to the belt (or pulley easiest for you to reach, depending on what kind of vehicle you're working on).
Rotate the tensioner back into position. Shine the flashlight on the tensioner pulley and examine the pulley face. If there's evidence of glazing on the pulley surface, the belt has been slipping on it.
Spin the tensioner pulley by hand and listen to it. If the bearing is good, the pulley will spin freely, straight and true, without any noise. If that's not the case, you have a bad bearing and must replace the tensioner.
Unfortunately, the end result of any belt tensioner noise is usually tensioner replacement. If you note any of these abnormal signs during your inspection, replace the tensioner. If none of these signs are evident, the noise may be issuing from another component and not the tensioner.