My mom's MacBook Air is making rattling noises when it is moved ever since I dropped it. Diagnosis? It dropped like two or three feet. The noise is like a rattling of a beanbag, but I don't know the source.
At the risk of sounding obvious, something inside broke. If you are the handy type and have the tools you could open it up following the directions for your model on iFixit.com or take it to an Apple store (or other Apple dealer who does repairs) and see what they say.

Also do not power on a laptop with something rattling around in it after you dropped it. It could make the problem worse.