Vibrating belt tensioner

In a 97 malibu when the car is idling it appears the belt tensioner is vibrating. After a few seconds the battery trouble light on the dashboard comes on. Any ideas?

On many cars, this one included, the belt tensioner will appear to be moving a bit while the engine is idling. As long as the belt is properly installed on the engine, it is doing its job. You need to have your charging system checked out before it leaves you stranded. The most probable cause of this is the alternator. The only way the tensioner will cause the charging system light to come on is if it’s not keeping the belt on the accessories. If it were slipping, you would hear it.