Scooter riders are always trying to figure out how to dial their scooters. Unfortunately, spring brakes have been a big problem in this area for many years. No matter what you do to them, they almost always rattle to a certain degree. The best thing you can do to solve this problem is to replace the brake with a flex fender style brake. I recommend the Phoenix Switchblade brake as it is extremely high quality, inexpensive, and made of spring steel for long-lasting performance. In addition to cutting down on the rattle, this style of brake will also make it more difficult to lock up your rear wheel so it offers some protection against flat spots.
Unfortunately, most decks are not built for a Phoenix Switchblade, but the process isn't too difficult. If you aren't comfortable using power tools or being precise with alignments, ask a parent or older friend to help you out.