I did some checking and apparently your car most likely has a timing chain rather than a belt. I used to work at a Datsun dealership where all the cars had timing chains and the first time I heard a loose timing chain I knew exactly what it was; the sound was unmistakable. Over time the chains stretch until the combination of that and the tensioner getting worn down allows enough flex in the chain that it rattles.
You won't be able to tell for sure on your car unless you take off the timing cover and examine the chain. It should be taut with no slack in it at all; if there is, that's most likely the source of the noise you're hearing.
To get any more detail on your problem, it would help a lot if you would add information about the year and engine size of your Grand Am.