I have never had to deal with a loose "rattling" truss rod yet, but everything I have read suggests that if the rod channel is a bit larger than the rod itself, wrap it in enough tape, or even plumbers thread tape, to make it fit snug in the channel. I would question (no offense to those who suggested it, particularly if you have done it and know it works) caulking because it is maleable and would simply give under the pressure of the rod and perhaps effect the efficiency of the rod. It won't rattle, but still may move in the channel requiring more adjustment to affect the same action. Once again, I have no direct experience with this issue, but everyone, including Melvyn Hiscock (if I am not mistaken, I don't have the book in front of me), says to wrap it to fit snug.
Peace...Rog