For this season at least, I think I can shed a little light on where the ravens hang out at night.....my yard!
I never noticed more than one or two ravens at a time in our part of West Cajon Valley (west of 138) in the eight or so years we've been here. But, this year, after the terrific Joshua Tree bloom we had this summer, we evidently have a bumper crop of seed pods on all the Joshuas. Since I also have an energetic young dog, we probably have less chipmunks and ground squirrels than usual this year. That meant that a lot of the seed pods weren't being eaten right away - they just hung on the trees.
Enter the ravens. First I noticed lots on them in our valley in general. Then they seem to hang around our Joshuas, eating up those seed pods. Now, as of this week, I keep seeing them on the treetops at night, and startling them into flight (rustling black wings in the dark!) as I walk the dogs slowly round with a small flashlight. Last night, for some reason, they were talkative, and kept cawing for an hour or two. From the talk, I'd guess that this may be the unkindness mentioned earlier, of about 50 birds or so! The Joshuas are about 20-25 feet tall, so the birds are in no danger from the dog or any stray coyotes, and breakfast is handy when they wake up, so I think that's why they seem to like it here.
The rattling/knocking noise they make is called "cronking", if I remember correctly!