Anybody who lives within 3 miles of the Woodland Park Zoo has heard this guy. Yup, he's one of the culprits, a siamang. They inflate their throats and let out a raucous chorus of whoops so loud it seems there's a slice of Sumatran jungle transplanted into the middle of Seattle. Two of them were whooping it up in grand style while I was there. I'm not certain, but they looked rather proud of their vocal abilities.