What's that sound?

San Francisco is a quiet city. Sometimes it's almost eerily quiet. When I'm walking to work in the morning, I always expect to hear all kinds of city noises, and there's nothing. Maybe it's the fog that dampens the noise and quiets the city. Walking through downtown during morning rush hour, I always expect to hear the kinds of noises I'd hear on my way to work in New York: horns honking, people talking into their cellphones or conversing with coworkers before going into their office buildings, people on early smoke breaks chatting with each other, street vendors selling coffee and bagels and tired New yorkers ordering their usual, the occasional jackhammer tearing up the sidewalk, police or ambulance sirens loudly bouncing off building exteriors, pedestrians swearing at cabs that get a little too close, and bicycle messengers ringing their bells to let you know they're about to run you over. But here, it's almost as if I have my earphones in and I'm listening to a silent song that blocks out all of the other noises around me...except that there really aren't any other noises. I mean, sure there is the occasional horn honking or person ordering a cup of coffee or homeless man asking for a couple of bucks, but beyond that it's unnervingly quiet. It's not as if there isn't anyone out - people fill the sidewalks on their way to work or wherever else people go at 8:00 in the morning, it's just that they don't seem to make any noise! It's very strange. I'm going to have to get to the bottom of this...