I started noticing a metallic rattling sound coming from inside my guitar whenever I moved it around. After opening it up, beyond a web of wires I discovered the source of the problem -- a small metallic cogwheel lying inside, attached to nothing. I have no idea, what it could be doing in there. Could it actually be a part of the guitar, that came loose, or is it a foreign body?
What you may have found, if it came from the guitar indeed, is a lock washer.
If that is the case, look for the nut or bolt which held it in place; a lock washer isn't a fastener by itself; it won't come off without some fastener also coming off.
It's probably not from a volume or tone potentiometer, because nuts and washers cannot come off unless these parts are removed. A washer between a potentiometer body and the inside wall of a guitar cannot come off unless the potentiometer comes off. You would remember if something like that happened with your guitar, right? (Did you have the guitar repaired or did you lend it to someone?)
It also wouldn't have come off the jack, because if that was possible without removing the jack, the part it would end up strung on the hookup wires. Those wires would have to break, and then there would be additional symptoms: your guitar not working electronically.
Rather, the part might have come from a circuit board, or from the assembly of some electronic part which is either self-contained on that part, or a fastener thereof that can loosen from the inside without external evidence.
This definitely sounds like hex nut which fastens one of the knobs or jacks into place on the guitar. I imagine it has also made the knob/jack which it was holding in place loose which adds to the rattle. You will need to screw this back onto the loose component and tighten with a spanner/wrench or pliers (not too tight!). This should stop the rattle and solve your problem.