If you engage in freestyle scootering, it's only a matter of time before your parts come lose or wear down.
Step 1: The Drop Test
To find out if there is anything off with your scooter, preform a drop test. Basically it is what it sounds, lift your scooter a few inches off the ground and drop it. If your scooter is is in good health (UNLESS it is a foldy) it will have a rich "dong" sound, similar to a bell. This state is called "dialed".
Step 2: Rattles
If your scooter fails the drop test, you will most likely hear some sort of rattle.
1). Heavy Metal on Metal: This type of rattle is most likely due to a loose headset. If your have a threaded system (See Picture 1) all you need to do is remove the bars and tighten the rings. If you own a threadless system (See Picture 2), and this includes integrated systems, you should only need to tighten your compression system (If you need help here let me know).
2). Squeaky or "Bongy": These noises are because of your break. If your deck has a flex break (See Picture 3) it will make a "bongy" noise. To fix it, scrape up the grip tape that's in front of the break, there should be anywhere between two to four screws in this area. Next, take off your back wheel. There will be bolts connected to the screws. Tighten these guys up and your ready to go. If you have a spring break (See Picture 4), you're out of luck. The springy noise they make is really un-stoppable. If it bugs you, replace it with a flex break.
3). Loud, Continuous Rattle (Only While The Scooter Is Rolling): This means a one or both of your axles are loose. Tighten them and your good.
Step 3: Un-aligned Bars
This problem won't be found through the drop test, but is pretty easy to spot. Most of the time this means that ether your clamp or compression system is loose. Tighten these guys and your golden.
If this doesn't work, then it's probably because your fork is to small for your bars. This is a uncommon thing, I've only seen it on customs and threaded systems.
And that is really all there is!