I've now had my car serviced twice for this issue twice in the past 3 weeks and it's as bad as its ever been.
I've already had both rear bearings replaced and diff fluid changed. I highly doubt it is a stone/pebble trapped between the caliper and heat shield, as some have suggested. This makes more of an inconsistent tumbling sound, as my guess is that would tend to make more of a consistent sound. I really tried to drive it off tonight, but it is now doing it continuously. I have no choice but to take it back to the dealer, but was hoping to get some help diagnosing it.
Here's a video: