Some buns are much noisier than others. I have a couple of buns who honk and quack as well as other things.They make their noises in fear, anxiety, excitement, happiness the full works.
It might be worth getting your buns chest checked out though, just in case there is a medical cause for it, but it is very possible that your buns are just vocal. Some like to communicate with us. It might also be that it is an effort cleaning whatever area they are cleaning. I have a girl who makes noises like that.
From the sounds of it though, chances are they are just loud bunnies.