If its rattling, why not just send it in for a new one?
I have three of them and all of them fit great.
Send it back.
Postage would cost more than the value of the knife. I probably got some factory seconds, because the ad said that the knife rattled, although it's quite safe in its sheath. Won't come out unless you pull hard. It doesn't seem to be very sharp. Actually I was disappointed with its edge and sharpened it a bit and now it's fairly sharp. I think that the heat gun is a good idea although nobody I know has one. Would a hair drier work?