Had that where I live, for the first year. San Francisco as well (it's windy here!) Finally after about a year of it, figured out it was the turbine ventilator on my next door neighbor's roof. She was a bit elderly and didn't seem to want to do anything. Luckily I realized I could reach it from my own roof. So I jammed a stick in it and it never spun again. Problem solved!
It's just an attic ventilator. And yes I know. My sanity outweighs the air quality of her attic. And I told her first.
(FWIW she's a very nice lady and has long since sold the house and moved away. I told her I had done that at one point and she just chuckled and didn't care)
