Strange cat sounds

Robin trills. It took a long time for me to come up for a word that matches the sound he makes, but that fits. He makes this sound in the back of his throat that is like a purr and a vocalisation at the same time. He'll periodically change pitch or punctuate with little 'meh' sounds and it sounds for all the world like he's carrying on a conversation. None of the other cats make the same variety of sounds. It's really strange.