Abstract: Molded resin vibration dampener is mounted on coiled tension window balance springs arranged within spring covers. A resin wall of the vibration dampener, shaped for fitting around the tension spring, has a slot oriented longitudinally of the spring, and the wall is flexible enough to allow the slot to be spread open so that the body can be pressed onto a mid-region of the spring. Resilient fins extend obliquely outward from the body far enough to engage an interior surface of the spring cover and thereby hold the body and the spring spaced away from the spring cover to prevent rattling of the spring within the cover.