I noticed our adult male cat doing this to calm the frantic meows of our younger female when she first entered our home. We kept them separate for the first couple of days (she stayed in a bathroom), and sometimes she'd meow at the door. The male got on the other side of the door and started 'cooing'. This calmed her down, and we decided it was time to let them hang out for longer than we had been up until then. So we let her out and and they've been best pals ever since!
It's definately a unique sound and one of many that is very important to a cats vocabulary!