I hear this all the time. I was just talking to someone about it. It’s not everyday but it’s frequent. I hear it mostly in the morning and at night.

It’s always the same length, about a second. It’ll come in pulses of 2 or 3 usually. Spread out a few minutes a part. It definitely sounds like an electronic sound. Almost like a sci fi satellite sound but deep in the water far away. Or maybe a far away whale mourning or something.

I just traveled for 3 months and heard it spread across 10 cities.

It’s very weird...

Power companies transmit a signal at certain times of the day through the lines. This controls devices that can switch between day and night mode. My ceiling fans will buzz when it's happening. Sounds almost like the old SMS interference you'd get with FM radio.

This is called Ripple Control, for anyone interested:

https://en.wikipedia.org/wiki/Load_management#Ripple_control

Thanks. I was struggling to remember the name.

Could you be hearing one of the many foghorns in the bay?

https://youtu.be/jJWd5eHeFY4

In the Mission I could occasionally hear foghorns, which made no sense to me. It was generally around first light in the morning. After quite a lot of digging, including making heatmaps of where ships parked, I traced it to Anchorage No 9, which is basically a parking lot for giant ships. You can see it when crossing the Bay Bridge.

More details and maps here: https://www.facebook.com/williamp/posts/10106726609193073

Pretty interesting hypothesis! I don't think so, though. It was a fairly electronic sounding boop, almost like a low sine wave. A fog horn sound has more acoustic qualities to it. Also, if it was a fog horn, I don't know why it would only be audible in Nob Hill and no where else I've been in SF.

The Bay foghorns are electronic tones; they don’t sound like classic/cartoon foghorns at all.

Wow. I went online and found an audio recording: https://www.youtube.com/watch?time_continue=12&v=CPUs2KnAyhU...

Yup, the tone at 0:08 is pretty much the exact tone I heard. Mind-blowing. Never would have imagined that's what a fog horn sounded like. I expected them to all be much deeper tones like the ones that come a few seconds later. I guess there must be weird acoustic qualities that make only the higher toned ones carry into Nob Hill.

If it's anything like the most common fog horn that the USCG uses across the country, it's a 273Hz tone (though I don't recall the waveform, most likely sinusoidal).

As of 8 years ago, they were using the FA-232 model fog horn, the circuitry was designed in the '70s and uses discrete BJTs on the boards to generate the tone and drive the "speaker".

It wasn't the foghorn was it?

I had this problem in college. Turned out to be the wind blowing over a small hole in a pipe, creating a whistle.

chirping smoke alarm? Saw a YT clip of a dude hunting for annoying beep with army of audio recorders, turned out to be battery depleted fire alarm under the stairs.

Considered this too, but there are two reason I ruled it out:

1. The beep was very long - maybe a second or two - much longer than smoke alarms. 2. I could hear it blocks away from my apartment. No smoke alarm chirp would carry that far.

Perhaps a UPS complaining that its battery is dying/dead?