Oh yeah, Murphy does a very good pigeon coo impression -- although not when using his litterbox. He graduates to the pigeon coo after he's been purring for a while, so I think of the pigeon coo as "advanced purring," like when he's really, really happy.
yup, i've seen this show up in many posts/threads before. the cooing like a pigeon consensus seems to mean your kitty is REALLY REALLY happy! i LOVE hearing this from our Angel. she does it when she gets her special treats and also when i put my arm around her during a nap sometimes.