I hear it all the time. Could it be a broken drip system emitter? Or a clogged one? This morning I stopped dead in my tracks. First thing I think is "rattle snake", but it was just the drippers turning on. Whew! Or maybe you've been watching too many horror movies.

My first thought was Cicadas. Second would have been a gas leak. Other than those two I have no idea. Interesting.

Wow, thanks, Kate and Susan!! I do feel better. You know there is a sprinkler system ... I don't think it is a drip system, but I guess any kind could make that sound. I didn't even think of that! (Plus I will do a search and make sure there are no cicadas here.) I looked again today and didn't see anything like a hive.

Cicadas don't really hiss. It's a hard sound to describe, but it's not a hiss--and not something your friend wouldn't hear.

What type of area is this? Suburban yard? Park? The "wild"? (Not with sprinklers...)

Hi Fori, the place this happened is just your basic OC suburb. I've seen occasional possums and skunks, and a few years back there were sometimes coyotes, although those seem to be gone. There could be a rat or two around. I think it was more likely to be an insect than an animal - but, I don't know all the animal noises there could be.

Meanwhile, the hissing hose theory is attractive bc this was so long and steady. I am not sure I think a steady hiss is that common in nature - at least, not that I've experienced.

@Need2SeeGreen 10 (SoCal), I'm in Riverside county on the edge of nowhere on 1.5 acres of rocky hillside. Our water has a lot of calcium and sometimes those sprinklers and emitters get clogged, thus the hiss. Drives hubby nuts, but what is worse is hose leaks. Irrigation lines lay in the hot sun many years and eventually they crack. Typically the leak is under some hard to access juniper or perish the thought, Bougainvillea! He goes in to battle with arm covers, for protection against thorns and heavy gloves. He's peacefully sleeping and there is a giant leak I saw this morning. I always take my phone with me outside on my morning rounds and take pics of the leaks so he can pinpoint them. If you know what time of day those sprinklers go on, and you walk over there, you might be able to pinpoint the hiss. Good luck.

Thanks for the suggestions, Susan and Fori! I haven't heard anything like that sound again. Otoh, we do have hard water, I am told. (I've never actually tested it.) I really should get under there and look.