Nope, the who in a great horned is a distinctive who with a deep tonal quality. This was definitely more of a child's slide whistle quality, more of a toot. I'm thinking it was a northern saw-whet as it most closely matches that tonal quality. Here is a link that might be useful: Listen to

Hey, those are native plants (out of focus) behind those birds :) Well, maybe they are...depending on where you are.

It's not a wren. My wren says "kiddeo kiddeo kiddeo." I listened to wrens' sounds online and it wasn't there. My wrens appear in plain view for me. Love them. Flibbidy jibbit is exactly what the bird says! It repeats it about 8 times and stops. It's always from high in the trees. I looked up the vireo's song and it isn't that. I've never seen a vireo where I live. Thank you both. I'm still trying to figure this out.

I'm going to take a shot at my own question after just now finding a similar post from 7 years ago... also in a propane tank, same kind of materials. Consensus was Carolina wrens -- which I love, and which have been around a great deal. Do others agree? Carolina wrens?