From my experience and what I've read in the past, it can indicate that they're anxious/excited/apprehensive/agitated. My gander doesn't like to be picked up, and when he knows I'm going to be picking him up to put him in their coop for the night, he does that little vibrating thing for a few seconds. Other times if I bring them their favorite type of plant to eat, they both shake like that because they're excited about it. It's perfectly normal goose behavior.
I agree with anxiety, I was just looking it up because I brought them into the office away from the fireworks and the goose was doing it. Now my Gander is doing it. I don't think they like indoors but they were freaking out over the fireworks. So they can deal with a little anxiety over a closed space as opposed to standing in a corner screaming for their lives.