"near the cap of the tower"? Do you mean the top of the speaker? I haven't noticed any rattling from my STS's, and the top where the MTM is seems to actually be the most acoustically inert surface on the tower. Have you tried emailing DefTech support?
Augustine's Law: "The last 10% of performance generates one-third of the cost and two-thirds of the problems."
"If it works the first time, it's not high end." ~ Harry Pierson