I received a ST300 when i warrantied a supertool, and the new one rattled the same way yours does. I emailed leatherman and they offered to mail out a new one along with a pre-paid envelope to ship the damaged one back.
Because of the logistics required(Had to have it sent to my parents place as i was out of the city, the envelope came separately, etc..) It took a few weeks to get everything sorted out, but i eventually got a solid working one in the end.
There are other areas where i'm disappointed with leatherman(I thought they included sheaths when the replace tools with different models. They do not, so now i have a few blasts and a ST300 that are naked and a few useless PST and ST sheaths) but in this instance they really did go above any beyond. Even checked to make sure the package was delivered(It said it was but i didnt have it. After checking with the post office, it turns out it was marked as delivered but put in a holding drawer waiting to be picked up. Had they not checked it prob. would have been deemed undeliverable and i would have been SOL)
If you're willing to pay the cost of shipping, i'm sure you'll get a rattle-free tool. May want to include a note asking the person to check to make sure the replacement isnt also rattling though, as its obvious from this thread and my experiences that they are more common than leatherman might like to admit.