Backpacker hears possible vocalizations near Mt. St. Helens witnessed in Cowlitz County, Washington

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I took a 3 day/2 night solo backpacking trip in Washington state, up the Ape Canyon Trail, around the base of Mt. St. Helens, and through the Muddy River drainage this past July 5th through 7th. On the first day of the hike at around 1 pm, as I made my way up the Ape Canyon Trail, I was surprised to hear a series of "whooping" calls, a series of 5 whoops in a row spaced at intervals of about 30 seconds to 1 minute apart, 5-7 sets over the course 10-15 minutes. The calls (bwaaahh-bwaaahh-bwaaahh) sounded as if they were emanating from the bottom of an oil drum, the tone being deep and bodily palpable, even though sounding maybe a mile or more away in the direction of the canyon, and somewhat north of the trail. It struck me as unnaturally deep and projecting (although not overly loud) and unlike any animal call I had ever heard before (I have hiked and camped in forest and wilderness areas for many years). I did not hear any other calls during course of my hike, save what sounded like a...