I hear them yipping and squealing like that all the time. No particular time of year. And they're moving.
I hear 'yotes making this sound fairly often, and like you said, they're moving. Usually it's the middle of the night and they sometimes sound like a siren in the distance.
I think it's simply chatter between them and helps them stay amped up and to a lesser degree, since they're roughly already together, is a locating sound, (the real locating call for coyotes is much different). Also, as Barry mentioned, that's probably only 2-4 coyotes. They have a strange ability to make a thimbleful sound like a rushing river.
BTW, KG, it's nice to see you posting here and there again.
