I've had two, a small Kostiner one and a large unmarked one that was used with a large process camera. Neither rattled, but both were very loud. They also vibrated. I would put a soft neoprene or similar pad underneath. I'd also locate the vacuum in another room if possible, using a long hose. It works _MUCH_ better this way as you won't hear it as much.
If they really rattle perhaps something has entered the vacuum chamber? I'm not exactly sure how these work but opening it up might be a suggestion.
Does it pull a good vacuum?