Apparently it's not something from the books, so Rafe was 100% sure no one would get it and so it wasn't a genuine offer. Which is a bit disappointing. Though one person was nearly right apparently. And he may do an official competition at some point in future. He also hasn't said which guess was almost right, which is just annoying!
Spoiler: Adding my guess EoTW spoiler I hear crows, I mean I can hear people, but it totally sounded like birds the first play - less as I play it more than once ETA ok just read the additional comments. Nevermind me
I'm hoping their choice of audio clip was intentional to mean they're dropping an actual trailer soon.
Rafe Judkins just did a Q&A on Instagram centered on adaptation changes. Here's a link to the whole things: Link. Here's my favorite two answers he gave in the whole thing, one hilarious and one pertinent as the show gets nearer.