I heard a strange animal noise while hiking, what was it?

Twice, in less than a week and in different locations about 25 miles apart (both in eastern Washington wetland areas) I was hiking with friends and heard the strangest sound. I can only describe it as a deep, sort of nasal grunting sound, like "Uuuuuuunh... unh unh unh unh." I thought it sounded like something a big animal like a moose would make, but I know it wasn't a moose. My friend thought it was a pacific northwest swamp hippo : ) And my mother thinks it was bigfoot. Maybe it was some sort of water fowl??? Any ideas?