I have only had my guys for... 4-5 months now, and I already am learning their language and odd mannerisms.
"Buk buk buk bak baaak!" Usually means "Something spooked me/I was just attacked!" But mine also use it when they want attention. I overslept by 25 minutes, the horror, I know, so I woke up to all my Silkies making a huge ruckus and I thought something had broken in and was attacking them!
I ran in, and they all went silent and stared at me as if asking "... Well?", so I sleepily grumbled about "crying wolf" and opened the door for the silly things.
They also have a quick and short "Du da da du du"-like sound they do when they see a small land-based predator such as my cat, and a growl-like noise when they see a crow or pigeon flying overhead. They also purr, which is like a softer, longer version of their "BIG BIRD!" warning call when being snuggled, when snuggling, dustbathing and sunbathing.
They also make "Graaah graaah" noises when running about with a treat, with the others sometimes making the noise in frustration. The rooster also makes that noise when 'playing chase' with me for when he is in a stubborn mood and wants to stay out later.