Are you talking about a bike with index shifting, or friction? Can you describe the rattling?

Chain noise can be a grinding sort of noise that happens when friction shifting is trying to move the chain to a larger chainwheel or rear sprocket. It can also be something continuous after you shift, coming either from the front or the back. This latter noise results from the front cage not being centered on the chain and rubbing the chain. In the rear it results in the rear cage not being positioned to feed the chain directly onto the sprocket. It can also be caused by chain or rear sprocket wear that causes the chain to hang up momentarily on the little hooks that you can see on a badly worn sprocket. This problem requires replacement of the whole freewheel or cassette and the chain, at the same time, and it happens on friction and on indexing systems.