Since the filmmakers didn't make it clear enough, you're guess is as good as mine.

I thought it might be a sort of "easter egg" for those who have read the books.

~Riella

It thought it would probably be one of the hot spots of the volcano or Aslan's roar in the distance that he heard. Perhaps Eustace is somewhat a believer of what Aslan is but doesn't know who Aslan is like when Lucy fell into temptation in the magician's house she could fully hear him because she fully believes in Aslan. So I thought it was either one of these.

I did find that weird. How in the world would that make sense if Eustace wasn't a dragon yet, and the old dragon wasn't likely ever there? I really don't know, but I think it might've been the GM or Eustace's mind giving him a warning.

I think it was the old dragon from the book. Maybe they intended to add the scene between Eustace and the old dragon there at first but left it out in the end...
But I agree, it's quite confusing.

I'd like to think it was the old dragon. I guess its up for grabs really. Maybe one of the film makers might comment on it one day.

Maybe the dragon was invisible?

I guess it was supposed to be the older dragon and they tried to show it without really showing it by having the dragon roar. But it didn't come across very well; I didn't remember that being in there and I've seen it three times. So it didn't make sense as to why the treasure was there if there was no dragon on the island and why Caspian would say "everybody knows dragon's treasure is enchanted" (or something like that) if there was no dragon there before.

Since the last 6 posts (starting with this one) in the General Movie Questions thread have to do with this topic which is still open, I decided to bump it as it is still relevant.

Like many of the film's elements, this is something that is not clear. I'll be the first to agree that a film should not be spoon-fed to the audience, but there's a lot of sloppy elements in the film.

My assumption was that it was always the older dragon. With all the gold lying around, one expects this to be the dragon's hoard. Eustace takes from the stockpile, not being one who believes in dragons, and the dragon's roar provides him with the fear that something is coming after him for taking the treasure. This also leads into the idea of Eustace's scorched clothing and diary along with the dragon capturing Edmund- that another dragon has killed off Eustace and is set to attack the crew of the DT. I suppose for one who hasn't read the books, this setup gives the viewer a sense of fear and mystery (what has happened to Eustace and what will happen to the rest of the crew with a dragon attack?).

However, there doesn't really seem to be anything to this island but a few hot springs and the like. Even the crew has a hard time getting food and water on a scarce island (how would another dragon survive here?). I doubt it was the mist, it has no real purpose except happening to show up when our heroes do something bad by being "posessed". Eustace's imagination? Could be. Somehow sounds become emphasized when we are afraid. Would Eustace be one to let his imagination (did he have one at this point?) get the better of him? I don't know.

I'm going to just imagine that it's meant to be the old dragon. That's what CS Lewis intended, therefore so shall I.

Maybe the roar was there to tell the audience something was going to happen? I wish they had shown what exactly was the source of the sound.

Yes, it could be the old dragon. Maybe the scene after that showing the dragon was one of the cut scenes? Or it could be an easter egg for fans. I believe it was most likely something to do with the old dragon.

It was probably not the mist, although Coriakin did say it could "take any form"...