Yup, the rattling would be lead pellets in the classic dead-blow hammer, good idea to explain that to the gang!

http://en.wikipedia.org/wiki/Dead_blow_hammer

The dead blow spreads the application force from one mass to another over time and across a wider surface, minimizing damage and making it a particularly effective way of giving something a wallop. [}] ]'> Got one, gotta get some acetone, nasty stuff. [] ]'>