If you have a long T15 screwdriver you can crack it open and see what's rattling around in there. Sears sells one, it's a red and black Craftsman. It's good to have around if you have a Plus. Two screws in the handle, two on the bottom by the ports and one in the battery compartment.
If it is a loose screw you'll probably want to get it out. Since this is your first Plus let us know if you have any trouble making boot disks, etc. There's tons of cool stuff you can do with your Mac. Have you tried Dark Castle yet?
I'm not sure how tightening those bottom two screws fixed your rattle but I'm glad it's fixed all the same! You can always buy a boot disk. That's a fun little machine, I'd give it a go if I were you!
You could always buy an Iomega Zip 100 to use as an external drive for your Plus. You wouldn't have to keep using floppy disks that way.
I booted it up a few days ago and was greeted by the friendly chime. Works fine, I ordered a boot disk from rescuemyclassicmac. Can't wait to start using it!