Both my banties and my standards make little cooing and "purring" noises when they are sleepy or content. I don't know if it is both male and female - I do know all my young female standards do this, so when I hear it in the banty brooder, I'd guess it was one of the female banties. If the male banty chicks are cooing or purring, I can't tell, although they well could be, too! The banties are all still so small at 5 weeks, sometimes its like a ventriliquist act trying to figure out which one is making noise!
I think the purring sound seems to be calling others to it to nap together, because when one of the standards makes the sound, a few will come over and quite often lay down with her.
Edited for clarification, twice, because it still wasn't clear!