Report 6655: Couple hears repetitive calls near Oregon Caves N. M

I and my now-ex-husband were hiking the Big Tree Trail after going on the Oregon Caves Tour.  We were about midway along in the hike, when we heard a loud, deep call.  It came from above us on the hillside, starting at our left. It started faintly, and picked-up volume as passed above us on the hillside, and then faded away.  It moved quickly... not as quickly as a bird flies, but far faster than a man travels. It was deep, and the quality was such that it seemed far away, yet was clearly audible. It sounded like, "whuh! whuh! whuh! whuh!"  The trail cuts through pretty dense forest, and we couldn't see anything. There were no starts and stops to this repetitive call, no slowing of movement judging by the movement of the sound.