What is it that wears out on these things? Is it the tensioner spring that causes the rattle or is it the bearing in the pulley? This has probably already been covered, but I couldn't find it with a search.

I replaced mine after 25000 miles it makes the most hiddeous racket, I tore mine apart and the bearings were worn out. Hasn't made a rattle since I replaced it.

Just replaced mine at 85,000, been rattling for a long time. Bearing didn't appear badly worn and looked like it should be simple to replace for very little money, I think. If it's the bearing I was thinking about replacing the old one and have a spare. If it's the spring then of course replacing the bearing would be a waste of time.