Well, it certainly is possible that there is a defect in your unit. Turn off the system and gently push the sub driver. It should move without any noise or feeling any roughness in the motion.
My assumption was that the sub was actually hitting against a physical object or causing the surround to snap taut from being "over driven" however this could also happen if something is amiss with the driver.
You might go on-line and find contact info for Edifier to get their advice on sending a substitute sub for you to compare.