You didn't really fully describe what's wrong other than a noise.

And this rattling is completely out of nowhere, it's never rattled like this before? Or it's had it before and it's just gotten louder?

Can you describe the rattling with any more detail aside from 'like a machine gun'? Any idea where it's coming from? Front or back of the car even? Does it rattle in sync with the sound of the engine turning over?

A bad alternator manifests totally differently differently than this. Typically your car will slow down, lights will dim, and your car will just die from lack of any electricity. Or it won't turn over at all (or maybe just go tick-tick-tick when you try). Although that latter is the same as a dead battery, which can be caused by an alternator not doing it's job of charging the battery while you drive.