Yep, I think all birdies do it. It's a happy-sleepy noise. My lovies all do it, except the new little one, though I think she let slip a couple of tiny grinding noises the other night after I put her to bed. My other girl grinds pretty loudly at sleepy time.
Well satisfied with having given mama a hard time - all in a day's work! LOL. Twinkie does it right in my ear when he starts to doze off while snuggling.