Possible late night vocalizations heard outside Redmond witnessed in King County, Washington

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I was lying in bed, trying to go to sleep, when I heard an unusual sound from outside through my partly-opened window. It sounded like a cross between a person moaning, a dog howling, and a cat growling (the timbre was human-like and in human vocal range, it was a high-to-low pitch sound like a howl, and there was a hint of growliness underneath that sounded like an angry cat, but much larger). It sounded almost like someone moaning "booooooo". The "b" sound was faint, but definitely present. Each vocalization was about 5-6 seconds in length, with about 5 seconds between each. It didn't seem very loud, but it also was not faint. I listened for several seconds, then got out of bed to peek out the window. My front yard is well-lit by a streetlight, but I didn't see anything. I have very bad eyesight, though, so after looking and listening for a minute and seeing nothing obvious, I went to get my glasses. I noticed my cat was staring at the window very intently, not moving. Normally wh...