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".