I'm intrigued by the show, though I'm hoping most of the "answers" aren't as sci-fi as people seem to be guessing -- it's a bit of a deus ex machina to just say "oh, it's something unnatural".
The crashing thing in the jungle sounds distinctly mechanical -- the sounds, when they occur, are pretty regular, and remind me of some kind of pressure release valve going off. Does anyone remember if the trees actually fall over, or do they more or less just go straight down?
I think the polar bear/comic book is a red herring. The simplest answer is that the plane was carrying a polar bear, presumably from an Australian zoo to a US one. The dog survived, after all, and the two would have been in the same area of the cargo hold.
As far as the message...I dunno. Easy enough to fake, if you wanted to. I'd certainly be hopping to find the source, though.
And the basics. They should be looking for sources of constant fresh water right now.
Looking forward to next week,
Nell.