Hi,
If your car has AC, you'll have an auto tensioner as well as a normal one. The rattling can be caused by two different things. The auto tensioner has a big spring-like thing on it, if this is worn (no way to tell though

) it will cause a rattling sound. The tensioner roll itself is mounted on a bracket with a pivoting point in the middle, the tensioner on one arm and the spring bit attaches to the other arm, the second reason for rattling is the bearing at the pivoting point. Take the assembly off the car (one bolt in the centre of the pivoting point) and check if the bearing has any play.
Cheers,
Niek