Hi Greig,
Ooh, you’ve sparked a neuron in my brain with that one! In the home-theater space, some people employ bass shakers, which are essentially weights strapped to beefy speakers. They are usually made for LFE (low-frequency extension) content from movies in mind, but if the optimum frequency is down there, it might be ideal. If it’s higher, you could dissect a smaller driver and strap a weight to that too provided it isn’t too special.
Seeing what others are doing with sand, I’d guess fine coffee grounds would like a fairly high frequency:
-James
