Yup... sounds like the top bushing of your belt tensioner damper is getting sloppy.
Here's a 25 cents fix. Insert an o-ring between the top bushing and the washer. This will pick up the slack in the bushing and stop the rattle. I did this 10k miles ago and it is still effective.
'93 300E 4Matic
'98 Nissan Altima
'00 Yamaha Road Star Silverado