It sounds to me like a smoke detector. Is there attic space above the garage? If so, it could be up there and going off intermittently due to dust on the sensor.

Hey I don't know about the smoke but you have nice rides ;)

Could it be the garage door seal around the garage doors vibrating from a wind gust? Something like that can create a similar sound, kind of like a kazoo. It doesn't sound like a smoke detector to me. It definitely sounds like air moving through something.

Ever figure out what was causing this?

It kind of sounds like a vibrating seal or loose sheet metal but the sound is so weird through your video mike it's hard to tell. I like the smoke detector idea. Unless you have a flock of angry metallic birds in there. :-o