open up the cart and take out the rattle... you will need a special little tool bit to open them up unless it is a standard flat head screw. You can pick up the special tool for like 5 bucks shipped I think on ebay. Cant think of what they are called exactly though...
Normally called a "Gamebit" screw driver piece, or just 3.8mm cartridge screwdriver bit...searching for something like that on ebay should turn up what you're after
The rattles I've had in the past are usually just a broken tab that holds the game cartridge in place, sometimes why they might not work as the cart is not making a good connection. Super glue works great for these issues.
Comments
3.8mm security bit tool
I dont know why I couldnt think of that...
~~NGD