Hello lilweeds - In addition to the rubber grommets you referenced, the belt cover is retained by a quick-release swell latch. The swell latch allows for quick and easy removal and replacement of the belt cover during routine maintenance, but the system is indeed prone to some cover vibration/movement, particularly when traveling over rough terrain. At the customer's option, the swell latch may be replaced with a threaded knob (installed by a Toro dealer free of charge). The threaded knob design still allows for removal and replacement of the belt cover without tools, but it provides greater clamping force, and in turn less vibration/movement of the cover during operation.
Thanks - The Toro Company