Every Tuesday at noon, an emergency siren goes off for about 15 seconds. When we 1st moved here, we were perplexed, then more curious when we realized that it happened every week, every Tuesday always at noon. We can barely hear it from our apartment, but it's very loud if your outside.
Here's a funny little video prepared by 72hours.org that explains it and its history:
Also found this:
Every Tuesday at noon, 65 air raid-style sirens around the city sound off for 15 seconds as a test for an emergency warning system. In a real emergency, the sirens will cycle continuously for around 5 minutes and will broadcast instructions instead of "this is a test." Those with access to radios should tune in to KCBS 740 AM, KQED 88.5 FM or a local news channel should this happen.
I think it's an interesting little tidbit about living in a big city.