He starts with a brux. Bruxing is when a rat grinds his teeth together. It's like purring. They are perfectly content and happy when bruxing.
The muscle that a rat contracts to grind his teeth actually runs behind his eyeball, so if a rat intensely bruxes, their eyeballs will bulge in and out. And, that's a boggle.
Here, Walternate bruxes and then revs up to a full -blown boggle. You can actually hear the brux in the video.