What is this sound?

I've lived in Houston for over a decade and I would always hear what I would think is an owl hooting. The hoots always have a distinct pattern that starts a little high and ends low.

Anywhere there are oak trees, I have heard this sound, especially in the Bellaire, River Oaks, Rice Village, Houston Zoo, Medical Center area. I don't have a recording of the sound, but it really is just a bunch of owl-like hoots in a soothing pattern. The hoots themselves are similar to the Great Horned Owl. The hoots that I've heard however are in a very distinct pattern. Perhaps, it is a call specific to Houston area owl??

Does anyone know what kind of owl creates this sound? Sorry for this oddly specific question