[Al Skierkiewicz]

Dave,
Generally there is nothing good about loose particles inside the battery. It is possible that someone dropped something on the cover over the pressure relief valves. Small pieces of plastic may have broken off.
It is also very likely, some of the internal plastic between cells has broken loose. If this is an AGM battery, there may not be any immediate danger. I would recycle before something occurs.
The pressure relief seals at the top of each cell I have seen are mostly rubber and would not rattle if they came loose.
Does the rattle appear restricted to the valve cover?