[Kalimac]

I'm not sure there is an answer to that one - none of the Fellowship would ever really be able to know whether it was the stone or not, though it's understandable that they'd guess it was, considering the tom-tap, tap-tom noise that started right after Pippin dropped it. Tolkien doesn't ever specify, possibly because it's not really *that* germane to the story what started it, just that it happened. Maybe it was the stone; on the other hand, maybe some nasty creature happened to overhear some random whisper or other and hastened off to spread the news to its fellows down below. If I had to bet I'd say the stone, but there's no way to know.

Incidentally I don't think it was Gollum; he'd been following them for a few days already, it would be a little strange for him to wait until they were almost out of there to spring the trap. Also, what good would it do him? Gollum is terrified of "Orcses" and wouldn't go seeking them out when there's no conceivable benefit for him. And there wouldn't be in this case; what would having the Fellowship be attacked do for him? It would mean that very likely the Ring would be captured by the Orcs, and good luck getting it back from them (since they have a highly un-Frodo like policy on letting pity stay their hands). Furthermore it could mean the possible destruction of his escape route, and indeed the main way out was destroyed. It's not clear how Gollum got out of the mines afterwards, but we can be pretty sure that it was more work than simply scampering over the bridge would have been. Gollum's best policy was to sneak along quietly and hope to catch Frodo alone and without fanfare.