Parrots grind their beaks when they are content. The grinding also helps them keep their lower beak in shape.....if you get the chance, look into their top beak, at the top, inside bottom.....it's ribbed, like a file...this is what they're rubbing/grinding their bottom beak on.
Also, if your bird rubs its beak along their perch or on their cage bars, they are filing down the outside of their beak.....it takes off loose beak layers & re-shapes/keeps beak shaped.....
Yeah...my guy does it when he snuggles up with me on the couch...or late at night if i happen to let him sit on my head...his favorite spot. I know hes happy, comfy, and sleepy when i hear that sound.
Yep. It's a sign of contentment. Niko spends every evening cuddled inside my hoodie for about an hour or so. As he quiets down for the evening, he'll first start tugging on my earlobe while making these cute baby sounds followed by the beak grinding.