1) The monster has appeared as a black smoke that can _float_ and _move quickly_
2) It was stated in a conversation between Jack (and Kate?) that no one should have survived the crash.
3) The plane broke up at altitude, yet managed to "land" relatively in tact. Heck, the engines were still running.
4) The Black Rock was found impossibly far from shore.
5) Metal hatch, metal ladder, and big, thick cable all part of the island.
6) Monster sounds can all be mechanical noices, from the screach to the clanking to the horn blowing.
7) So far we have a ship, small plane, and airliner and at least three groups of survivors.
8) When the monster is "attacking" we see the trees disappear. Nothing is dropping out of the sky and we have seen a deep, deep hole (listen to the dynamite explosion) that Locke almost got pulled into and the monster rushing out of.
9) We have a hatch going down a deep, deep hole.
1) The monster is a swarm of small black things/insects/little machines.
2) The machine sounds could be the monster itself or some support system (like a metal door opening, electical generator, etc.).
3) There is a vast underground complex where we have cool/sinister machines run by Dr. No/AI/noone. We know something's down there - the light from the hatch window.
4) For diabolical reason X, the monster can pull a plane out of the air and grab a ship and move it inland. The monster takes care to keep as many people alive as possible. A airliner may have been too big and fragile to pull down in one piece.
5) (This is a stretch) The only person the monster killed was the pilot. He would have know what was happening to the plane as it was coming down. No Witnesses.
6) The monster attacks/pulls in/no escape any vehicle that comes close to the island.
7) Locke's work on the hatch alerted the people/entity below ground of the hatch's discover, so the ladder was removed to prevent access.
Chew on that.