Mysterious water noise found and fixed
Friday afternoon an SPU crew showed up to fix the water leak. By that time it was getting to be a gusher. While the guy who originally look at the problem thought that the water main on our street itself might be busted and thus need to be replaced (requiring either the street or the sidewalk to be torn up), the leak actually was in the spur pipe to our house, making it a much smaller job. I was lucky that the leak was on the street side of our meter, as home owners are responsible for problems between the meter and the house--but this one was the city's problem.
With the leak fixed, the weird noise in the house has disappeared. Yay!