It seems that every time I venture neer the dragon roost at Mt. Anthor, I hear a distinct rumbling sound, complete with screen shake and controller rumble. I've killed the dragon there already so it can't be the dragon's doing. Any idea what this is or how I can find out?

Maybe by asking in the spoilers forum- for a lot of people (me included), we did not kill the dragon there yet. You could be giving away a heavy plot point.