Wish I knew what it was too. I've noticed this both times I've taken my car up to Canada. Just on really long highway drives it appears. I thought it was my bad cv joints, but I've since had those replaced and it still does it. Seems wierd because after the car sits for a bit, it won't do it anymore.
Besides that, mine is more of a bunk-bump-bippity-bunk-bunk.
Ryan