Mine only ever do it when they're about to be aggressive... I haven't noticed any happiness at all about it. Every time the shaking starts I get the skin on my arm grabbed and twisted!
When a chicken gets close, the Mallard cross hen will start vibrating her head and then BAM she gooses the chicken right on the back. I just wrangled 4 of them yesterday to cut some bands off now that I have the flock all sorted and the extras sold. 3 of the 4 seemed alright with being held and handled. The 4th... started vibrating and then I got grabbed. Not a game, I don't think.
So when they do it, I move my arm and start petting them behind the head. The usually diverts the attack. When I had to wrap the call duck boys for angel wing, I was the brunt of their frustration. They were not pleased, and started shaking the minute I picked them up. I even got a bruise from one grab, he just wouldn't let go of my forearm!
You know, as sweet as they are, they can be quite mean. I get far better treatment from the chickens! The chickens are all sweet, will run to me whether I have food or not. The ducks... always looking at me with that one distrustful eye and acting like I'm trying to poison them when I give them a new treat. Ungrateful things. Don't they know they're twice as much work as the chickens?
Good thing I like the challenge of being their friend. I finally got a drake to eat from my hand yesterday without any shaking or meanness! That only took 3 months.
They were so sweet as day old babies, running to me and calling me mom. What happened? haha.