My own "OK, what was that?"
For around the last 3 months my wife and I have been hearing a mysterious sound at roughly the same time of the morning perhaps 2 or 3 times a week, while using our respective computers in the living room. It sounds like it is coming from outside as the dining area door is usually open to the back yard.
The sound lasts about 1 second, and is best described as pretty close to a cat hissing when provoked...ssssshhhhhhhh, with a back of the throat element to it. Pretty much white noise with a little amplitude and tone shift.
For the first several weeks, I would run into the back yard to see if I could spot the culprit, to no avail. We speculated, perhaps a new squirrel noise? a neighbor's patio door? something failing in the house? garbage truck air brakes? pranks? Poltergeists???

etc.
Last week, my wife told me she heard the sound come from my computer area when I was not at home, completely on the other side of the room from the patio door to the outside. Finally, I myself heard the sound come from my own desktop area, and realized what the source was!
My iPod Touch was updating, via wifi, the notifications for a game app called "Trucks and Skulls". Instead of using a musical tone, like most apps, Trucks and Skulls uses a whooshing sound which is pretty loud coming from the tiny speaker.
The iPod, along with phone and car keys, usually resides on the dining room table, near the door, except when being charged by my computer in the evening!! That's why it originally sounded like the noise was coming from outside. Difficult mystery finally solved! Wow!