This might sound like a stupid suggestion, but have you tried switching it off? If the noise stops when it's off I would suggest it's probably a mechanical noise rather than organic
Also, check the outside of the building where the A/C connects. If there's not a hole there large enough for an animal, it's pretty unlikely to be an animal!
Rats and mice can make themselves pretty flat, but you'd probably have heard that squeaking in there before the rattling. If it's a rodent you'll need to get it out before it stinks the place up. The other option would be a spider, but if it's rattling then it's already dead so I wouldn't worry about it