hi chiba,
we use a similar method, stick a plastic drinks bottle, half full of water, half
way in the ground. Insert a childs windmill, the sort you would see in a fair
ground. The noise as it spins and twists conducts well thu the water into
the ground.
We also hang plastic bottles, with a few pebbles in, from the top of the
garden posts, they swing around, hitting the post from time to time.
I know they dont work when the wind isnt blowing, before one says so.
Its low tech, 'green' and its cheap.
Regards
EricG