Why do mourning doves vibrate?

Lately I've noticed the mourning dove nesting in our planter has been vibrating a lot lately. It's throat visibly and silently vibrates. I've even noticed it's tail has been vibrating as well and it's causing the plants in the planter to shake too.

Currently there's two babies in the nest with it so I assume it's some form of contentment like a cat purring?

When I first go outside, it's usually like an unblinking statue. Once I'm sitting down and eating or playing music at a reasonable volume, it's quite animated, curious, blinking and vibrating constantly.

I also thought maybe it has something to do with producing crop milk since this all started sometime around when the babies hatched but I can't find anything when I search for more information.