The VW Passat serpentine belt tensioner is a cast aluminum assembly. It has the spring loaded tensioner arm and the idler pulley integrated in one part. Typically if you begin to hear squealing or grinding noises from your engine the pulley has gone bad. Poor belt tension can also cause your serpentine belt to slip, which is clearly marked by extremely glossy spots on your belt, this can lead to premature belt failure. If you find that one of these parts is failing the entire assembly must be replaced.
When you replace the serpentine belt tensioner pick up a new serpentine belt. This is also a prime time to inspect your timing belt. The timing belt is behind the serpentine belt, if it fails you won't only be stranded, you can cause major damage to your cylinder head and other internal engine components. Save yourself the headache and take a peek while you are there.