I does sound very similar but this one was actually explained at some point in the story (or session play?). It was a giant cave claw Falcon Clan was using and their digging caused ramblings. Sounded way too big for what it was in the end but yeah, back then the main focus was on the books' "backyard" (follow in fellowship's footsteps with Grey Company, explore regions), not original events and new "unknown" threats which is the case now. So no doubt there is a better, satisfying explanation for the most recent reoccurring mountain tremors