Get some of disc magnets and stick them under the base of your can to stop the ball-bearing from rattling. You can find really strong magnets online. Though I've tried to use regular magnets before and my cans would still make a lot of noise. You don't need to be quiet. If someone is paying enough attention to hear the noise of your rattling cans and cares enough to do something about it, you're already fucked.
Try using a plastic bag, if you use a backpack you might risk losing it if something happens. Plus it is easier to prevent sounds if you hold the bag in your hand. If it's unavoidable uses an over-the-shoulder type bag because they sit in there better than a (taller) backpack, and you can keep it vertical while you run/ climb / jump / sneak.
Make sure all the cans are upright with caps on, to prevent accidentally spraying the inside of the bag. At any fast food drive threw you can grab a cup holder and simply place the spray paint cans there for a support box. Also use a hoody or an old rag to squashed on top to stop or in between cans to stop them clanking together.
