A loose tsuba is a common problem with production katana, this is because the hole in the centre of the tsuba has been cut large enough to fit any of the dozens of blade tangs that these companies make. This is good for the companies economy and works out well for enthusiasts such as ourselves, because this is one of the factors that has keeps the cost of producing functional swords down.
Unfortunately although a poorly fitting tsuba isn't a problem in and of itself, it can be indicative of other issues. If you can move the seppa or tsuba around then thats too loose in my opinion. There are two types of movement that a poorly fit tsuba can exhibit, 1) shifting up and down or 2) from side to side and there are a number of ways that you can address them.
Replacing the seppa
You can tell if the seppa isn't thick enough for your configuration if you can move them up and down at all. They dont need to move much at all in fact it may be barely perceptible but if you can do this then you will need thicker seppa. Sometimes this happens when the rest of the fittings are changed, for example when the fuchi is changed and is thinner at the top than the one that was previously fitted.
Now seppa aren't all the same, chinese ones tend to be rapidly stamped out with little consistency. they're also quite thin. The best bet is to get some that are thicker than you think you will need and if you need to, file or sand them down a little. As an alternative, if theres a lot of wiggle room there, you can always fit multiple seppa to get the same effect although this may look messier.
Closing up the nakago ana
The nakago ana is the hole in the tsuba that the tange of the blade, or 'nakago' fits through. If you have a tsuba that has been handmade rather than cast, then you may want to stamp the edges of this hole that the nakago goes through. This has the effect of closing up the size of the hole, fitting it more tightly to the nakago. Note: This will _not_ work with cast tsuba as it will break, chip or shatter. The other consideration is that handmade tsuba are a lot more expensive than cast ones and so you should only do this if you are absolutely sure that youre not holding an antique and that you're happy adjusting it yourself. better safe than sorry.
In order to punch the hole smaller, you will need to position the tsuba on something hard, taking care to only place the area that the seppa normally covers onto the hard spot so you don't damage the worked metal. Using a hammer and a punch, tap at the metal on the edge of the nakago ana. This will flatten it slightly and close up the hole a little. You will have to take the tsuba to the sword and check the fit as you proceed. Go slowly, check often.
This is usually more than enough to stop any movement but bear in mind that this modification only addresses lateral movement.
another section? (If the gap is large enough, brass strip can be used to fill the gap but you will need to glue and file)
Using Polyform to fill small gaps
As a temporary fix and when the amount of movement is very small, you can use a material called polymorph which is easily obtainable from hobbyist stores. It is a heat melting thermo plastic and when placed in very hot water, goes soft and malleable readay to be formed into any shape you want. As it cools it returns to its normal consistency which is a rigid but not brittle plastic. You can use this to fill the gap between the nakago and the tsuba in any direction. When youve got the plastic where you want it on the tsuba, push the parts of the sword carefully together and wait for it to cool, it doesnt take long.
Once it's cooled, check the fit to see if it moves or rattles. If it doesn't simply take it apart, remove the piece of plastic and heat it up again. In order for this to work, you will have to pay close attention to where the tsuba moves and where the fit needs support. With this very untraditional fix, you will find that YMMV but I have found it to work wonders.
So briefly in conclusion
These few methods are the easiest that I have found to quickly address the issue of a rattling tsuba. They are by no means the only ways that work or indeed the best ways but I have found them to be effective and easy enough for anyone to implement. The tsuba, in my opinion should never rattle or move. It should always be tight between the seppa and the rest of the fittings. On a 'working' sword however, especially one used by backyard or freestyle cutters who are often making mistakes as they try to push their own boundaries, these things can take a beating and often work loose over time. This is why you should always check your fittings regularly and stay safe.