In Berry's words:
"As dusk became dark night, something approached camp from a ridge above, rapping on wood or rocks as it came, and when it arrived, two voices that I could discern, it vocalized, and the sounds carried through the trees as I have never heard human voices carry every before or since. And it whistled, a clear, beautiful whistle like a bird might make, between its kind, and at one point back and forth with us."
According to Berry, this went one for some time: "This encounter went on for nearly an hour and a half, and another followed on the second night, and there were other encounters I can attest to later that season. I was able to get reasonably good tape recordings of the sounds and interaction, and we cast several of the foot impressions, both in pine mat and snow. I looked high and low for evidence of the joke, including searching the others' belongings while they were away hunting. I wasn't a novice investigator of facts, but I came home stumped, basically with nothing to write about until the story unraveled by itself or I helped in with further research and investigation."
The strange vocalizations became known to some as the "samurai sounds," due to an almost uncanny resemblance to the expressive style favored in old samurai films like those by Kurosawa.
Watch this scene from Kurosawa's classic "Yojimbo" and compare.