Mine does it too. If I press on it right at the top where the little storage bin starts I can get it to stop. It only seems to do it on rough roads, or in winter.
In summer I have something in the driver side B pillar that clunks when I go around a corner. No idea what that is. Dealer claimed that it didn't do it for them. I think they just couldn't find it or didn't feel like checking.
"THIS is my BOOMSTICK! You can find it in Aisle 5 of the sporting goods department... Shop Smart, Shop S-Mart!"