During our recent visit to Arizona, we spent 3 days in chilly, but gorgeous Sedona. While taking a short hike around our hotel, we heard some gurgling and cooing and looked up to see this pitch-black raven bobbing and weaving, making a very unusual noise. I know that ravens and crows can make dozens of different sounds, and they are notorious for mimicking other birds and animals as well, but I had never heard this variation before. It's sort of a cross between a mourning dove and a sand hill crane. Could it be a mating call of sorts? He kept it up for several minutes before flying off. I've also posted a video below which includes a short burst of it. (Hope it works for you.)
UPDATE: I think I've come close on this one... it almost sounds like he is mimicking the call of the Sand Hill Crane, which is making a very strong come-back around the US once again. When they fly high over-head, their regular call is quite muted and is similar to this raven's 'churl'.