It's either a spooky electronic voice phenomenon (EVP) of Reagan himself, or The Romney Unit was receiving answers from an advisor. Listen for the whisper, "Raise taxes."
From Paddy at Cliff Schecter's house and John Amato's version here.
What the hell was that? It could've been one of the other candidates, but why the hell would they want to prompt Romney Unit 3000-21?
UPDATE: The more I listen to this, the more it sounds like another candidate thinking out loud or something. One of them has a weird habit of making little outbursts -- laughs and such. Remember the "hoo-hoo!" thing?