Here is a link with some tips on how to take care of a FW:
{
http://thefeatherweight221factory.com/ }
Here is the page inside that gives you tips on how to tune them:
{
http://thefeatherweight221factory.com/page10.php }
Now, the reason the bobbin case rattles is because the two little springs the finger sits between are bent, or out of adjustment. They should be tight enough to the finger that it doesn't rattle. They can be adjusted a wee bit. But, be very careful because those springs are brittle and will break if you try and bend them. And ... they are not available as new parts anymore. If you break one, you'll have to find one from a parted out machine. Please don't ask me how I know this.
To adjust the little springs, loosen the screw and move them towards each other as needed. They won't move much and if they are as close as they'll go, don't force them.
Joe